Okay, so, for all those getting the error messages, I've got some info from my emails with Marriott CS:
First got this message:
We're sorry but this account is not eligible for this program. Please try again, or contact support for additional assistance.
Answer: you have to have an ACTIVE Facebook account, that is with at least 1 friend. As my account was essentially set up to just enter contests and such, accepted one of my friend requests and tried again. At which point I got the second error message:
Your email address was not provided. Please check your privacy settings. Contact us if you believe this is incorrect.
Answer: Apparently, those with new FB accounts need to wait 24 hours after activating (or friending their first friend, in my case) before registering.
Interesting point: you apparently can use the FB account of a friend/family member if you wish, but only one MR account can use each FB account. I do not know, however, if a MR account can be linked to more than 1 FB account.