First, on an Intel Mac you
must use the Universal build of aMule. Running the PPC build under Rosetta has known problems.
Second, make sure you have a good server list. Go to
http://ed2k.2x4u.de and use one of the server.met files they provide.
Lastly, you may need to set up a hostname for your Mac. There are instructions toward the end of this thread:
http://forum.amule.org/thread.php?threadid=9753.