I bought the Masterpiece version of the first Myst a few years back. I went to re-download it, but there's only the option to download the manual and bonus content....? Under "My Game", there's no option to download the game? Help?

Exact same thing is happening to me right now.

Same here.

http://www.gog.com/forum/general/games_showing_but_downloads_not

It's not just Myst:ME. The staff is working on fixing that.

It's back; downloading right now.