Instructions at the website: http://www.physics.umanitoba.ca/~english/2013fallphys1830/bwimaging/

Clicking on the GIMP link on the  bwimaging website
--> goes to  http://www.gimp.org/downloads/

For MAC:

Click on 
                     Download gimp-2.8.4-nopython-dmg-1.dmg

When the blue fills the download bar in your browser (top right corner) click on the down arrow.  In the menu that appears (i.e. your Downloads folder)  double click on the gimp .dmg file that you just downloaded
This gives  Gimp.app in a "Gimp device" window.  

Open a new finder window and go to Applications folder. Drag the GIMP.app into the Applications folder. 

Double click on Gimp.app and answer the "wizard's" questions:
   - Open  (... want to open it?)

If it doesn't open right away, double click again on Gimp.app