If Presence is telling you you've entered the wrong user name or password when you try to log in, make sure you are entering your Mac's user name and password, not the name you chose when you set up Presence - that's just used to identify your Mac.
Your Mac user name is the same as the name of your home folder (displayed in the Finder's sidebar with a small house icon beside it).
The password is the one you enter when you install software updates, etc.
If your Mac password is blank, Presence won't let you log in. This is to keep your Mac secure from unwanted intruders.
You set your Mac's password in System Preferences - under Users & Groups (Lion) or Accounts (Snow Leopard). If you don't want to enter your password when your Mac starts up, turn on automatic login in the Login Options section.