Hi there mate. I havnt tried to download Demo Monkey, and on the Apple Dev Portal, looking for it, i get its a retired document. Apple dont use it anymore, as they had if for development with Xcode 3.1.2 and OS 10.6, which is now 3 years behind us. Xcode 4 and 5 are so much more advanced now, and it gets easier and easier to Develop now with Xcode, once you find what they have changed and where they have put things.
On your other question, read my post above #48, i believe IMHO anyways that C and Obj-C are closer to each other than C and C#.
I found it easier to learn C 1st then get into Obj-C. Im still learning, but started off wrong, thinking Obj-C was the way to go, but as i dug deeper, i found the experts, said its better first to get a understanding of C before you even touch Obj-C, and once you have done that, you will know why they said to tackle C 1st.
I have know idea what C# is like to write, but if your experienced in C# then you should understand why the experts say get into C before Obj-C.
Mate i been a Mac man for years, and only have Windows on Bootcamp, because its the only way for me to play a RC Helicopter and Plane Simulator , but if you REALLY want Monkey, i found it here (outdated link removed). There is a $99 pro version and a Demo so try before you buy.
Really i would learn to write in Obj-C because with Xcode and the Dev Portal, you have a LOT of resources at your hands, and even 2 free Specialist Apple Engineer help as well. With the cross platform 3rd party Apps, your on your own, with maybe Stack Overflow as your resources. Its well worth the $99 even when learning as its a wealth of knowledge behind those Dev Portal Doors
HTH
Cheers
PS And i just downloaded the Demo and opens just fine. It will be the Monkey.app in the Folder once you unzip it.