Here is the Mac OS X's compiled libraries of XMPP wireshark plugin currently developped by our GSoC student Mariusz (http://code.google.com/p/xmpp-dissector/).
It will work only Wireshark development release (version 1.6.0). Note that stable version (1.4.7) is not compatible as it lacks stuff to compile the xmpp plugin.
So you can get the Wireshark 1.6.0 at http://www.wireshark.org/download.html, copy the xmpp.so to Wireshark.app/Contents/Resources/lib/wireshark/plugins/xmpp.so and start wireshark as usual.
Please note that this is a work in progress. We will try to update the Mac OS X plugin binary in the future to reflect project progress.
Note for Linux users, compilation instruction can be found at http://code.google.com/p/xmpp-dissector/wiki/Compilation .