I'm assuming the is sticky stuff on the platen and the platen is otherwise undamaged. Were it mine, I'f probably take the rubber bottom piece out try some mineral spirits with an old wash cloth. Seal has become D&K (I think) and they have a tech support site: http://www.forframersonly.com/techHelp.asp which might get you headed in the right direction. Bill BArber
I'd use something a little stronger than mineral spirits, say acetone or naptha, but only with good ventilation and nitrile gloves. If a simple wash won't do it, you may be looking at a need for serious solvent action, which you may or may not be comfortable doing at home. If so, lacquer thinner will clean just about anything off anything, but it's nasty stuff. Again, don't forget the gloves and ventilation.
I picked up a government surplus press a few months ago. It was filthy. I tried all the solvents I had - no luck. Finally used an orbital sander with a fine grade paper. Polished up real nice. The platen was aluminum and a quick wipe afterwards cleaned up the last of dust/dirt. I have a feeling the were mounting printed material rather than photos.