Does it write anything to console.log (or, if you launch from the Terminal, to the terminal window)? Does it produce a crash report (~/Library/Logs/CrashReporter/amule.crash.log)? When it's frozen, you might try sampling it from Activity Monitor (double-click the process and the press the Sample button) and posting the sample output here.
Without more information, we won't be able to figure out what's going on.
Also, there are known problems with BOINC-based programs (such as SETI@Home) and the distributed.net client. Try disabling those.