Regular bus service runs from Labin to Opatija, not from Rabac.
From Labin take bus to Rabac:
http://www.travel-file.com/labin-rabac-buses.htm
Any bus from Pula to Rijeka stops in Labin and Opatija :
PULA – RIJEKA
5.15, 5.30 (except Sun), 6 (except Sat and Sun), 6.45,7, 8 (except Sat and Sun), 8.30, 9, 10.30 (except Sun), 11,12 (except Sat and Sun), 13.30, 14.15, 15.15, 16.15, 16.50, 18, 18.30 (Mon, Wed, Sun), 20, 21.30, 23.30
pulainfo.hr/en/autobus.asp
autotrans.hr
For latest info call informacije – Autobusni kolodvor Labin: telephone no 855 220