[MonoDevelop] Can't Run/Debug. Huh?

[Tess Snider]

A friend of mine installed MonoDevelop on his Mac, and no matter what
project he loaded (even the simplest Hello World application),
MonoDevelop does not provide the ability to Run or Debug the app, even
though it builds fine.  These options are greyed-out in the menu, and
hotkeys don't do anything, either.  At the time, I showed him how it
was working fine on my Windows machine.

However, last week, I loaded up one of my MonoDevelop projects on my
Windows machine, and I was having the SAME PROBLEM.  I thought maybe
my project settings had become messed-up, so I -- again -- tried a
minimalist Hello World app -- and no matter what I did, I could not
get back my Run/Debug, in spite of the applications building fine.  My
Debugger AddIn is still right where it should be, and everything else
seems to be in order, so I'm stumped.

Is this a common problem?  Does anyone have any advice for troubleshooting this?

Tess