Don't know the 28i, but in principle, If there's a red light and it goes out then you've probably won! the filling loop is a flexi-pipe which may or may not be already fitted between two stoptaps somewhere either below the unit or elsewhere in the pipework. If it's 'around' but not fitted, there should be two adjacent taps with open ends. Fit it and then open both taps and watch the gauge. If you overfill it then reduce the pressure by bleeding one of the radiators.
Wes, December 2009