Have you checked your iPhone to ensure that FaceTime and iMessage have not signed you out? If they have, sign in again and allow your iPhone to communicate that it is set up. You should then get the option back.
Failing that, reset your Apple ID password to refresh your account and then resign in, across all devices with the new credentials. This should jump start the preference.
Or, it may be a bug... 'quelle surprise'. You take your chances with beta's and I speak from experience.
I've just rolled back to 10.10.1. having found all sorts in 10.10.2 14C81f. Stuff that I simply could not be a**ed to iron out and report on. This sloppy coding is becoming tiring