They do run a lot of promotions. You just have to sign up for it, on their site, and then it's automatic from there. I'm currently finishing up the last one. It's pretty simple.
You play poker and earn points. Earn X number of points, get $Y.
For the summertime one, for example, earn 50 points playing (fairly easy to do) over a week, get $10. Get 100, make $25. Get it to 150 points, make $40. This is actually slightly better than the one I am currently finishing. I needed 100 more points (200 total) to get from $25 to $50. This time, I only need 50 more and hit the cap.
As for being worth it, I would say so. Since they don't have rakeback, you really should be taking advantage of as many promotions as you can. I have not finished this week (so I don't have final numbers) but I have made $200 in bonus money (will make $225 when done). During that time, I paid over $290 in rake. That's like 69% rakeback. Of course, everyone's mileage will vary. But, it's well worth it.
Is how they do it a little sleazy? Yeah. But it's not terrible either. Probably benefits the micro players more than those who play tons and will easily go well over 150-200 points in a couple days (let alone a week).