SUO senses to see if you TSVN installed. So if you do, SUO will indicate this. After that, you just select 2 files from the popup. After that, SUO just sends the 2 filenames to TortoiseMerge so you can see the graphical diff.
Best you can do is just install TSVN and try it. Really very simple for diffing. Using it for SVN can be hard at first to setup, but ultimately makes version control effortless. If you want to see the 1st version of ScriptUO, I can easily recompile.