Why can't I log into my Apple Account to install OS X Mountain Lion?

So I got my MacBook used. Someone else install everything for me because I know nothing about computers. I got a bad virus so I reset the computer to factory settings. Afterward I tried to install OS X Mountain Lion again but when I go to sign into my apple ID, nothing happens when I press sign in. How do I log in so I can install it!? I don't know what other information to put because I don't know much about this sort of stuff...