I just experienced the exact same problem, and spent about two hours trying to find an answer online, until I finally got it to work.
I got stuck at the same step that you did, both when I tried it on the Setup Assistant initially, and later when I gave up on Setup Assistant, made a user, and tried Migration Assistant.
What finally worked was this: In the Finder on the new Mac, I opened the disk that contained the Time Machine backup, and double-clicked on the backup file itself (a single file with a name ending in ".sparsebundle"). This caused a new disk volume to mount itself on the desktop, and suddenly Migration Assistant recognized THIS volume as a Time Machine disk. Everything else has proceeded nicely, and the restore is humming away as I type this.
One other caveat, which I learned in my search... Apparently if you are restoring via Migration Assistant, you can't restore to a user account that you are currently on, and you can't restore to a user account with the same name as the one you are restoring, without replacing the former. So what I had to do (Since I had already made a new user on the new computer with the same name as the user on the Time Machine backup) was make a temporary fake user with admin privileges, log in as that user, use Migration Assistant to restore the Time Machine backup and choose the option to REPLACE the same-name user on the new computer, and then later I will delete this fake user.
Surprisingly complicated, it seems... And unbelievable that this would be the way Apple intended the process to go. I still am not sure how I could have gotten this to work from the start within Setup Assistant, which obviously you're supposed to be able to do,