Here's a walkthrough of the Tinkercad -> 123D migration for those of you who are still having trouble! This has worked for most people, but if your models are still missing, give this a shot.
We've also gotten reports of Safari and IE giving people the most trouble. We're aware of that and are working on another update to go out tonight, in the meantime, try Chrome or Firefox. However, if you've exhausted this process and you're still having issues, give us an email at email@example.com.