The only references I see to GIT in Aptana Studio 3 (the standalone version) are under preferences where the only field is to identify an executable (do I need to do that?) and importing a project, where it asks for a repository uri. I see nowhere to configure user information for a particular repository, and, if I need to install a plugin, nowhere to do that.
First question and, apologies if this is obvious, but have you installed Git on your system? On my Mac, I have/had to install Git first, before I could use it in Aptana.
Once you have verified that you have Git installed on your Win, Mac or 'Nix system, open Aptana and set the executable in the Git preference pane that you note in your question.
From this point, you should have Git options from Aptana. In the App Explorer view, there's a sprocket icon that, when clicked, will now give you Git options. Also, if you right-click on a file in your App Explorer, you'll have Git options available, too.