I know once I ask this question someone will point out the glaringly obvious to me, but can anyone tell me why in the scene in GF2 where Michael is in Cuba and and all the men are having cake for Roth's birthday and Michael is talking on and mentions about the rebels he saw and how one of them blew himself up, why does Roth get so angry with him for bringing it up?

Short answer: Roth was in a hurry for Michael to fork over the $2 million, so he could then have Michael killed. That's the part of the scene where he showed his anger ("The $2 million never got to the island").
Longer answer: Roth intended to have Michael killed at Tahoe after Anthony's party because it was a perfect opportunity. When it failed, Roth went to Plan B: lure Michael to Havana (where Roth was the most powerful gringo) with the promise of turning over all his Cuban gambling interests to Michael; wheedle $2 million out of Michael as "a gift to the President" (Batista); have Michael killed by Batista's military guards; keep the $2 million or give it to Batista as a gift from Roth, not Michael. But Michael never intended giving Roth the $2 million because he knew Roth was behind the Tahoe attack, and that Roth planned to have him killed in Havana. He deliberately delayed having Fredo bring the $2 million to Havana in order to make Roth impatient and show his hand. Michael used his observations of rebel activity as a way to keep Roth waiting for the $2 million.

I was hoping that you would read this and reply, thanks Turnbull!

Roth was talking about parceling out his Cuban gambling interests to the others who were present at the cake cutting.

I think we can reasonably infer that since Michael was "investing" in Cuba, some or all of the others present were investing as well, and Roth preferred that Michael not express his reservations in front of them, lest they get ideas about backing out of the deal.

You could infer all of that from Roth's words. But he wasn't sincere, imo. I think he went through that whole charade about giving pieces to this one and that one to provide an alibi for the imminent assassination of Michael. In front of a bunch of witnesses, Roth made it appear that he was planning a well-thought-out, orderly transition of his empire to his heir-apparent. So, if Michael turned up missing or dead, Roth could say, "Who, me? Why, I loved the lad like a son. Just the other day, I anointed him my successor...just ask all those sterling characters who were at my birthday party!" What about the Lakeville Road Boys, Eddie Levine of Newport, the Penino brothers, and the people back home whom Roth promised pieces of the action? He mentioned them, imo, for two reasons: First, to show he'd carefully thought through his succession plan (reinforcing that he had no motivation to kill Michael). Second, to spread suspicion: If Hagen, Neri, Rocco or another Corleone ally came looking for vengeance, they might think that one of the people slated to share Roth's empire with Michael got jealous and greedy, and whacked Michael. Clever fellow, Roth!
I have to admit, plawrence, that it hadn't occurred to me that Roth might have tried to wheedle money out of the other guys for the privilege of sharing in his empire. That'd be even more clever.