By default, Mac OS X has core dumping disabled. You can enable it, but it is easier to run amuled in gdb in the first place.
I have seen the "Could not unlock mutex" message here, but it only appears once and then amuled works.
Have you ever run aMule with this user account before? I'm wondering if you have a pre-existing ~/.aMule directory. There have been recent changes to aMule's source so that it now uses ~/Library/Application Support/aMule as its configuration directory instead. But the code to relocate a pre-existing ~/.aMule directory to the new location has not received thorough testing, and it may be a problem with that.