::Scratching head::
Um, this was a really simple question to answer and all of you guys seem to have this thing for making something simple, really complex. Don't mean that in a mean way, just a truthful one.
Okay, the reason none of the keystrokes "work" besides the enter key when you use the 'su' command is because they are working, but the keystrokes are not echoed to the command line for added security. This is because if, by chance, someone is looking over your shoulder and all they can see is asterisks (Leave it to the MS Dictatorship of Mr. Gates himself to be too stupid to figure this one out), then even though they might not have seen what you typed, they now know how many characters your password is which would narrow down their cracking time when they later try to attempt to gain access to your system. Oh how much easier it would be to know that your password was exactly 6 characters long than it would be to have to wonder if your password was anywhere between 4 and 400 characters long. Muuuuuuuuuuuuuuch bigger dictionary.
So, all anybody had to say is that, once you type 'su' and hit enter, and you come to the 'password:' prompt, just type your password and hit enter, even though you don't see anything come up on the screen when you type and everything should work out just fine.
Next time - RTFM