ok, sounds like you are missing the Picasso96 stuff to get the HDMI running as a graphics card.
the ApolloOS image is here http://apolloos.weebly.com
- i'm downloading it myself now, and have ordered a 32gig CD card and usb adapter to drop the image onto, so I have no idea about that.
base things you need installed.
Picasso96 selecting the UAEgfx option during install.
then the SAGA drivers.
then you should be able to find Picasso96 under "prefs" and add/remove screenmodes if you need to, but you should find a stack more screen modes in screenmode prefs that are the SAGA screen modes.