Hi,
The FIRST PAGE of the user manual, titled "LOGIN IN", available online, says and explains all:
http://www.fop2.com/documentation-userm ... gin-in.php
Use your extension and password as defined in /usr/local/fop2/fop2.cfg to login. If you use the standard FreePBX install, you will login with your Extension Number and Voicemail Password. In order to perform actions with the panel is mandatory to login with some extension, as that extension will be the originator of the actions you perform.
If you want to modify passwords, use the fop2admin manager module, or you have the option to configure things by hand, but if using freepbx I do not recommend it at all.