I have create a new extension. It appears a error which means that the helper data class not found.

My config files are as follows.

appcodelocalETLizenzetcconfig.xml

<config>
    <modules>
        <ET_Lizenz>
            <version>1.0.0</version>
        </ET_Lizenz>
    </modules>

    <global>
        <helpers>
            <ET_Lizenz>
                <class>ET_Lizenz_Helper</class>
            </ET_Lizenz>
        </helpers>

        <blocks>
            <ET_Lizenz>
                <class>ET_Lizenz_Block</class>
            </ET_Lizenz>
        </blocks>

        <models>
            <ET_Lizenz>
                <class>ET_Lizenz_Model</class>
                <resourceModel>ET_Lizenz_mysql4</resourceModel>
            </ET_Lizenz>

            <ET_Lizenz_mysql4>
                <class>ET_Lizenz_Model_Mysql4</class>
                <entities>
                    <Unternehmen>
                        <table>ET_Lizenz_Unternehmen</table>
                    </Unternehmen>
                </entities>
            </ET_Lizenz_mysql4>
        </models>

        <template>
            <email>
                 <ET_Lizenz_confirm_new_customer_template translate="label" module="ET_Lizenz">
                    <label>Template to confirm a new registered customer when using Unternehmenslizenz</label>
                    <file>et/lizenz/confirmation.phtml</file>
                    <type>html</type>
                </ET_Lizenz_confirm_new_customer_template>
            </email>
        </template>

        <default>
            <ET_Lizenz>
                <confirm_new_customer>
                    <identity></identity>
                    <template>ET_Lizenz_confirm_new_customer_template</template>
                    <copy_method></copy_method>
                    <copy_to></copy_to>
                    <cron_time>0,1,0</cron_time>
                    <cron_frequency>0</cron_frequency>
                </confirm_new_customer>
            </ET_Lizenz>
        </default>

        <resources>
            <ET_Lizenz_setup>
                <setup>
                    <module>ET_Lizenz</module>
                    <class>Mage_Eav_Model_Entity_Setup</class>
                </setup>
            </ET_Lizenz_setup>
        </resources>

        <events>
            <events>
                <customer_save_after>
                    <observers>
                        <ET_Lizenz>
                            <class>ET_Lizenz/observer</class>
                            <method>customerRegisterConfirm_after</method>
                        </ET_Lizenz>
                    </observers>
                </customer_save_after>
            </events>
            <events>
                <customer_save_before>
                    <observers>
                        <ET_Lizenz>
                            <class>ET_Lizenz/observer</class>
                            <method>customerRegisterConfirm_before</method>
                        </ET_Lizenz>>
                    </observers>
                </customer_save_before>
            </events>

    </global>

    <frontend>
        <events>
            <controller_action_predispatch>
                <observers>
                    <ET_Lizenz>
                        <class>ET_Lizenz_Model_Customer_Observer</class>
                        <method>customerRegisterConfirm</method>
                    </ET_Lizenz>>
                </observers>
            </controller_action_predispatch>
        </events>
    </frontend>

    <crontab>
        <jobs>
            <customer_confirm_email>
                <schedule>
                    <cron_expr>18 0 * * *</cron_expr>
                </schedule>
                <run>
                    <model>ET_Lizenz/ET_Lizenz_Model_Customer_Observer</model>
                </run>
            </lieferant_email>
        </jobs>
    </crontab>

    <admin>
        <routers>
            <adminhtml>
                <args>
                    <modules>
                        <ET_Lizenz before="Mage_Adminhtml">ET_Lizenz_Adminhtml</ET_Lizenz>
                    </modules>
                </args>
            </adminhtml>
        </routers>
    </admin>

    <adminhtml>
        <layout>
            <updates>
                <ET_Lizenz>
                    <file>et/lizenz.xml</file>
                </ET_Lizenz>
            </updates>
        </layout>                          
    </adminhtml>
</config>

appcodelocalETLizenzetcadminhtml.xml

<?xml version="1.0"?>
<config>
    <menu>
        <customer>
            <children>
                <ET_Lizenz_Unternehmen translate="title" module="et_lizenz">
                    <title>Unternehmenslizenz</title>
                    <sort_order>500</sort_order>
                    <action>adminhtml/et_lizenz_unternehmen</action>
                </ET_Lizenz_Unternehmen>
            </children>
        </customer>
    </menu>
    <acl>
        <resources>
            <admin>
                <children>
                    <customer>
                        <children>
                            <membership_company>
                                <title>Mitgliedsunternehmen</title>
                            </membership_company>
                        </children>
                    </customer>
                </children>
            </admin>
        </resources>
    </acl>
</config>

appcodelocalETLizenzHelperData.php

class ET_Lizenz_Helper_Data extends Mage_Core_Helper_Abstract
{
}

And I am getting the following error.

Fatal error: Class ‘Mage_Et_Lizenz_Helper_Data’ not found in /WWWROOT/256574/htdocs/app/Mage.php on line 547