If you attach to a game that uses mono, you should see a new "Mono" menu on the main Cheat Engine form. This should have an 'Activate mono features' item and a 'Dissect mono' item. The 'Search' item in this image is a link I'm working on.
Activate mono features will inject the appropriate DLL for the MonoDataCollector from the autorun\dlls folder where you have Cheat Engine installed and set up a pipe to communicate with it. The source code for the MonoDataCollector is in the folder of the same name in the "Cheat Engine" directory when you check out the source code.
Dissect Mono (shortcut Ctrl+Alt+M) will open a form that lets you browse the mono classes available.