did i read somewhere that you can use a credit (say from paperback club) to get a dvd from that club?

I think it takes 3 of PBS credits to get 2 DVD credits. I've always been switching the other way--when I got some DVD credits, I brought them over here because I'm really not much of a movie watcher. I got 3 PBS credits for every 2 DVD credits.

You have to transfer the credits between sites to use them on the other site.

Yes, what Barbara said.  You can also use your credits in SwapaCD and there, they are 1:1.  You have to open a new account with each site, though, just so you know.  The icons for both sites are at the top of this screen.

I just transferred some PBS credits to Swapadvd.  It takes 1.5 PBS credits to get 1 DVD credit.  It's really simple to transfer them-under your main account page there's a Transfer Credits Option then you just follow the prompts.