How should I seal a metal window frame to a cinder block wall?
Urethane or 100% silicone caulk. Urethane for the best bond and toughness, silicone if you can't find urethane. Silicone is not paintable. 

Clean up all loose material first. Press in foam rope as a backer if needed, leaving room for at least 1/4" of caulk.