Did you make sure that you added an Offset to tell the game which parts of the map coordinates corresponds to which part of the level?
By default the offset is (0|0), means the center of the radar is at the origin of the editor. If you created your level too far away from the origin and didn't specify an offset, you might not see your map.
But maybe you also made another mistake somewhere.. I am new to RadarMaps in UE3 as well.
That seems to be the old way of handling the Minimap. It now is handled in scaleform. It is called as a flash file in a .swf form. Check out the file udk_minimap.fla and udk_hud.fla in the flash folder. You may have to download it.