You can make maps via export to engine and they are playable ingame. However, saving a .cry file will return "Save Failed." If you can't save .cry files then you can NEVER reopen them in the sandbox.
Either this is an error with my install, or it's what Crytek gave us. An official response would be most welcome.
THE FIX
Rename your "Crysis? 2 Limited Edition" folder to "Crysis 2 Limited Edition" and it should work.