Based on Tomas Petricek's instructions here and here.

1. Download and install Mono for Mac here (Open the .dmg and run the .pkg): http://www.go-mono.com/mono-downloads/download.html
   
2. Download and install MonoDevelop for Mac here: http://monodevelop.com/download
3. Download, unzip and install F# for Mac here: http://fsxplat.codeplex.com/releases
4. Install the F# MonoDevelop language bindings
   1. In MonoDevelop, click Tools | Add-in Manager and click the "Install Add-ins..." button.
   2. Next click the "Repositories..." button, then the "Add" button.
   3. Make sure the "Register and on-line repository" radio button is selected and enter http://functional-variations.net/addin/ and click "OK".
      
      
      
   4. Close out of the "Add-in Repository Management" dialog, expand the "Language bindings" node, check the "F# Language Binding..." and click "Forward".
      
      
      
   5. Click through default options in the add-in installation and restart MonoDevelop after it completes.
   6. Now you should see the F# project and file templates when creating new file or project.