Have you compiled app ? I suppose yes.
If you want to change UI the package is com.openkm.frontend in trunk version and in OpenKM 4.0 es.git.openkm.frontend
The class you are looking for is called mainmenu.java ( you can find into widget/mainmenu/ )
Be patient with code, in few days you'll can understand how it's structured the project, for example all frontend UI is in that package ( es.git.openkm.frontend) , servlets are too defined in frontend/server ( we recomend you be familiarized with GWT ).
Hope it could help you.
If you want to make some enhancement o collaborate with us in developement I recomend you contact with us using web contact form. We're interested in collaborators, and make your live as easy as we can trying mantaining your code enhancements stable for newer OpenKM versions.