Joe's answer is on the right track, but possibly misleading (sorry Joe, I think you're thinking of Fontographer :-).

The inner path and the outer path need to be going in different directions. There should be a small gray arrow along each path indicating the direction of the path. If they are going in the same direction, they will both be solid. You need to change the direction of one of them by selecting the path and right-clicking or control-clicking to get the contextual pop-up menu. Select "Reverse contours" from the menu.

Paths within paths always need to go in opposite directions. For every path inside another path, you need to reverse the direction again (for instance for an O with a dot in the middle).

The direction of the outer path depends on what kind of font it will be. For PostScript, use counter-clockwise for the outer path. For TrueType, use clockwise.

>I was thinking of FontLabographerStudio. <

Is that an Osmaroid company? :-)