I'm running on a Mac with Vidalia, so the setting file (you can find the location of it in the Vidalia settings dialog's advanced tab - but it's probably ~/Library/Vidalia/torrc) ends with:
Note that the country code is GB and not UK, which a couple of posts/articles online misleadingly state.
There's another problem with the recent Mac builds - the geoip file is in the wrong place, leading to log messages like the following:
[Warning] Failed to open GEOIP file /Applications/Vidalia.app/share/tor/geoip. We've been configured to use (or avoid) nodes in certain countries, and we need GEOIP information to figure out which ones they are.
That was solved like this (from the terminal):
mkdir -p share/tor/
cp Contents/Resources/geoip share/tor/
Then stop and restart tor.
Update: StrictNodes should now be StrictExitNodes.