Hi sgreen,
Here's a link to the Celtic Manor CTTH.
When you press the "Play Now" button in the game client on the left of your screen, you'll see the main menu. Click "Closest-to-the-hole" to test your skills. These courses are for CTTH challenges only and don't need to be unlocked. Click "stroke play" for the four full 18 hole stroke play courses. Click "match play" to play in a multi-player match.
Remember Green, this game is, and always will be free to play. You can upgrade you're clubs at you're own discretion when you reach the required level to unlock the club.
Off you go! : )