I started using TortoiseSVN the other day and found that when I would attempt to checkout a repository I couldn't do it because the "ok" button was grayed out and I could not find good info on what might cause such a problem, but I finally did figure it out.
I found that adding a slash "\" to the end of the checkout directory field after I chose the directory fixed the problem.
EXAMPLE: If you have a folder called c:\test and you browse to it because you want that to be your checkout directory the field is then filled with the value "c:\test". You will notice that the "ok" box remains grayed out. The program actually requires an extra slash at the end of the directory name that isn't put there by the browse control but can easily be added manually after the fact. So if you change the value to "c:\test\" the ok box will no longer be grayed out and you can continue your checkout operation.
NOTE: This problem was found on TortoiseSVN version 1.5.5, I have no experience with similar problems with other versions.