Heya Arnimon
Do you have the latest version (v4.06) of FSUIPC installed?
If not, go here and get it:
http://www.schiratti.com/dowson.htmlThis download is for the base version which will enable some base features including radar screens. I gave a donation and received a registration code which unlocked more features.
If this doesn't work (or if you already have FSUIPC) then you may try to look at the gauges themselves. If the HUD and Radar Screen were initially designed for FS2004 using FS 98 base gauges they may not be compatible with FSX. When you select the aircraft in FSX do you get the compatibility message for the specific gauge? What happens when you click "Yes" to make this a trusted gauge?