If your activation code that you get in e-mail upon registering, looks similar to this. You won't be able to click it and log in.
http://www.mysite.com/forum/index.php?action=activate;u=19981;code=c483346f5d
What you'll need to do is highlight the entire thing (including the numbers on the end) by dragging your mouse over it with the left mouse button down.
Once it's highlighted, hit your "ctrl" key and your "c" key at the same time to copy the link.
Click your cursor in the address bar of your browser, and then hit your "ctrl" key and your "v" key to paste the code in.
Then hit enter.
Also...Just the activation code can be copied and brought over here. Click on "Missed my activation email" under the log in info on the left side up there and it'll give you a window to enter that code.