As far as I am aware the only way to find a 7Ds shutter actuations on a Mac is by installing GPhoto2 from source. Although this is relatively straight forward with a package manager, it is still rather involved for an average user and involves a 4GB plus download for Xcode. As a result I have put together a simple little app based on a GPhoto2 framework to do this.
You can get it here:
http://southwest16.com/use/shuttercounter-1.0.zip
Just connect the camera with the USB cable, open the app, and click the refresh button. It may take several seconds (just as when using the command line GPhoto2) so may beachball, but it will eventually show either the shutter count, serial, and name fields or an error.
The app should run on OS X 10.6 (Snow Leopard) or later, and I believe the shutter count is visible on the following models (or later in same range): 1000D (Rebel XS/Kiss F), 550D (Rebel T2i/Kiss X4), 50D, 7D, 5DII, 1DII.
I have tested it successfully with a 7D on Lion (10.7.5) and Mountain Lion (10.8.2), although you must quit the app before the camera disconnects (either sleeps, turned off, or physically unplugged) else the OS X USB controller will cause the program to crash.
