Plugins are modules which can add functionality to Pymol.
Plugins on OS X
The standard OS X Pymol application, MacPyMOL.app does not run with the Tcl/Tk interface which is required for plugins to work. However, a quick renaming of the program from MacPyMOL.app to PyMOLX11Hybrid.app makes the application run as an X11 application, and plugins are now available.
To rename the executable, right click (or control click) on MacPyMOL and choose "Get Info" in the Panel. Change the Name & Extension to PyMOLX11Hybrid.app. This name can also be changed using the mv command in Terminal.app.
Once this change is made, half of the program will show up under the X11 icon, and half will show up under the MacPyMOL icon.
An alternative to renaming MacPyMOL is to download the PyMOL installation using Fink:
fink install pymol-py25