There are loads of posts on support forums and kicking around the web about not being able to log into FMF as it won't support the Apple ID that people have. I had the same issue, and here is my workaround.
On my iPhone 4 I have two accounts set up. One for the iTunes store (someemail@personaldomain.com) and one for iCloud email/contacts etc (someemail@me.com). I was trying to use FMF with the someemail@me.com address but kept getting an error telling me that my ID was incorrect and that it must be verified.
Solution: Go to appleid.apple.com and click the link to manage your ID. Log in with the ID you use for the iTunes store and, when you are in there, add in the other email you wanted to use with FMF. You will be sent a verification email to that address.
Once you have done that log into FMF using your iTunes store account. Hopefully it should work and it will have any invitations/requests filtered through as well as your email is now linked.
In my case that meant that I had someemail@me.com as my primary email and someemail@personaldomain.com as the secondary email on the iTunes store account. The invitations that had been sent to my iCloud (someemail@me.com) account showed up just fine.
