Sorry I haven't responded earlier, I've been pretty busy this last week.
It turns out that many of my Adventures contained assets that I obtained through manual download (rather than subscriptions or sporecasts). So when I switched computers, I lost access to those creations.
Fortunately, I tried dragging the .pngs into the editor as suggested, and it worked perfectly! Those missing creations are back, and I can finally resume work on my GroxQuest saga.
The only things that didn't return were my awesome snapshots (though they are still in my MSC folder), but I can always take more.
Thank you guys for the help!