Hi, we've used MonobjC for the exact same thing.
I'd highly recommend it.
There are others out there (cocoa#, ...) but this one seems to be the most complete.
It is very actively maintained, which lots of excellent support if you need it.
Check out the samples, fairly easy too.
The other option is indeed MonoMac.
It's the new, officially supported version, but, as far as I know, it's not exactly 100% finished yet. Expect some bugs/unimplemented stuff here + very samll userbase currently.
For the moment, if you want everything to work, I'd still recommend Monobjc.