Hmm...
Do this: Go to system preferences -> accounts. In the left side, you see a list of the user accounts on the computer (even if there is just one, i.e. you). Underneath your name, there should be an access level -- it should say "admin," "standard," or "managed." If it does not say "admin," then there should be an unchecked box on the right side when you click on your account, that says something like "allow this user to administrate this computer." Check that box.
If that's not the issue, then we have to figure out what else it might be. 😉