I'm going out on a limb here, but:

I'm guessing that Ola didn't promise Fredo anything specific--because he didn't have to. Here's the dialog:

"Johnny Ola bumped into me in Beverly Hills -- and he said that he wanted to talk -- he said that you and -- and -- ROTH were in on a -- a -- big deal together. And that there was something in it for me if I'd help 'em out. He said that -- he said that -- you were bein' tough on the negotiations. But if they could get a little help -- and close the deal fast -- it'd be good for the family.

MICHAEL

You believed that story. You believed that.

FREDO

He said there was something in it for me -- on my own."

I'm guessing that Ola did "bump into" Fredo in Beverly Hills. He promptly steered Fredo to the nearest lounge, plied him with drinks, all the while commiserating about how rotten it was that deserving older brother Fredo got passed over for the Donship. So Fredo, getting more drunk, and more self-pitying, was ready to listen when Ola told him that if he gave Roth and him "help" with the Cuba deal, there'd be something in it for him. That "something" was the Donship--but Ola didn't have to explicitly say it, he let Fredo infer it.

Fredo obviously wasn't in on Michael's Cuba discussions and couldn't pass on inside info to Ola that might have earned him his reward. But, when Ola asked him to open the drapes to Michael's bedroom, Fredo finally saw something he could do to get his Donship. He might have asked Ola why he wanted the drapes opened, and Ola might have said, "We just wanna take your brother away somewhere and talk some sense to him"--just what Fredo might want to hear to assuage his "conscience" (and might have led him to his "you guys lied to me" remark later). But, the only way Fredo could even have a shot at the Donship was if Michael was dead--and Fredo knew it without being told.