It's possible that it's not assigned in your controller configuration. If that's not the problem, then we'll need a bit more information about what's happening (or not happening, as the case may be).
In what way isn't it working? Are you unable to use FSS moves at all or are you having problems toggling between FSS movesets for players who have multiple FSS abilities?
It may sound like a stupid question, but are you using the right stick or the face buttons (or indeed, a controller or the keyboard)? In NBA Live 07, the FSS moves are mapped to the right stick rather than the face buttons as they were in NBA Live 06.
What do you mean? I've already played this game before and decided to install NBA live 2k9 but its a big pain in my PC's memory so I go back to NBA live 07. Should I redownload it again? because I've already deleted the installer.
Yes, that's because the functions assigned to the face buttons (or in your case, the keys) do not change when you press the Superstar Trigger; in the normal case they're only activated by using the right analog stick, which by default does not have equivalent functions in the keyboard configuration.
Basically, the problem is the limitation of the keyboard controls in NBA Live, so you've got two options: invest in a dual analog controller, or search the Forum for one of the mods that binds the right analog moves to the keyboard. Running a search for keyboard bind right stick (or something along those lines) should bring up some threads with more information on the subject as well as a few download links of pre-altered configuration files.
I don't have any links handy but using those search terms here in the 07 section, you should be able to bring up a couple of threads. It wasn't discussed too long ago so you might even be able to go back through the last few pages, the 07 section isn't as active as it used to be.