GeboGebo uses the external GNU command line tool diff.

Linux administrators normally don't have to touch anything for GeboGebo using the diff command line tool correctly.

If you run GeboGebo on a Windows host, you don't have the diff view supported by default.

Please get http://unxutils.sourceforge.net/ the diff.exe from there, since it works exactly as the Linux pendant does.
Copy it into a place of your choice (propably the ./bin/ directory) and ensure the gebogebo.conf option Diff= points to it.