if that's the genuine Macvision part and not just a generic, then all you need to note from wally's post is -
Find a 9 volt AC 300mA wallwart with the right size plug to go into the serial adapter and you're good to go.
If all you have is a generic DB-9 to MiniDIN-8 - do not plug that into the Macvision. It's wired wrong, and you (maybe) risk damaging the Mac or Macvision.
Or, find someone who is tech savvy and make up an adapter. There's enough info in wally's post for that. There must be DC regulators and rectifiers in there to produce +5V/+12V from the AC input, and a power socket for the wallwart. Presumably therefore this is a box with cables coming out of it, not a straight cable.