Greetings...
There's a couple of things you can try out.. Open up a console window (that's sorta like a dos window). Do the following command which hopefully outputs something useful:
~ $ whereis amule
You should get something similar to this:
dracco@epitaph dracco $ whereis amule
amule: /usr/bin/amule /usr/X11R6/bin/amule /usr/bin/X11/amule /usr/man/man1/amule.1.gz /usr/share/man/man1/amule.1.gz
We're looking for the amule binary (executable) which, in my example, is the one with "bin" in the path.
If you get nothing, it did not install or it just didn't install correctly. If you do get a path to the binary, try executing the amule binary with the entire path. If that works, you need to add that path (/usr/bin in my example) to your system user profile defined in ~/.bashrc or ~/.bash_profile, depending on distribution.