I've been looking around on SO to see when to vote on tag synonyms. But I cannot find the answer.
Could anyone of you help me out?
Unfortunately (or furtunately, who knows), one needs to have a total answer score of 5 or more on a specific tag to be able to vote for the synonym. Here "total answer score" is total upvotes minus total downvotes. From your profile page I can see that you have not answered any questions on the apple-push-notifications tag, thus you can't vote.
However, it should show a notification about that for you, like this: