Terrorists yes, ISIS no.
This photo is making the rounds of the blogsphere accompanied by the claim that ISIS members on Twitter are claiming he posed with them. I have yet to see a specific citation of that claim even being made. Not that it matters.
That isn't to say that McCain wasn't posing with terrorists and terrorist supporters. He was. But they were members of the Free Syrian Army, which had strong links to the Muslim Brotherhood and other Jihadist groups.
McCain is standing next to General Salim Idris, the front man for the FSA, who wasn't actually in Syria. The man to the right of him is Mouaz Moustafa, a Palestinian Arab lobbyist for all sorts of groups who supports Hamas.
So terrorists yes, ISIS no.
It's not inconceivable that one or two of the men in the photo may have drifted through various organizations and ended up being aligned with ISIS. But they weren't ISIS at the time.
Common sense alone tells you that Idris would not have allowed armed ISIS members near him and an American VIP.
Obviously McCain should not have been posing with them or supporting any of them to begin with.