If you are using Geometry Friends under Linux or Mac you might not have access to Visual Studio to develop and debug your agents as assumed in the Step by Step Installation Guide . However, it is also very easy to use MonoDevelop which can be easily installed alongside mono in both Linux and Mac.
To use the competition pack and develop/debug your agents using MonoDevelop you can follow the Step by Step Installation Guide with the following difference:
Step 4: this step is not possible so you will just have to be careful to build before running the game. Always press F8 (Build All) prior to F5 (Start Debugging) so it is all built before running.