Magento 1.8 – Fix Cron Job Not Working

Have a cronjob in my extension config.xml file but seems do not run and function don't load

What is wrong here??

config.xml :

<?xml version="1.0"?>
<config>
    <global>
        <models>
            <mymodule>
                <class>Mycompany_Mymodule_Model</class>
            </mymodule>                         
        </models>
    </global>
    <crontab>
        <jobs>
            <mycompany_mymodule>
                <schedule><cron_expr>*/5 * * * *</cron_expr></schedule>
                <run><model>mymodule/observer::myFunction</model></run>
            </mycompany_mymodule>
        </jobs>
    </crontab>
</config>

And in Observer.php

public function myFunction() 
    {
        ....            
        ....
        ....
    }

Also added cron to server in Directadmin panel :

*/5 * * * * /usr/bin/wget -O /dev/null http://www.mydomain.com/cron.php

and

*/5 * * * * wget -q http://www.mydomain.com/cron.php

When I run cron.php from URL directly, it work and function load!