This thread should have all the information you need. Basically, you write a post, submit it, and then a button with Dice Roller or something appears and you can use that to roll the dice.
Actually, I think that's good and useful for pbp. Better than Invisible Castle, especially since the site kept going down for a while and it couldn't even be used...
When I decided to roll 1000 dice on this [Doing 1000 was my being curious to see how many dice can be rolled], it hadn't occurred to me at the time that I wouldn't be able to get rid of them. Else I would've only done like 100.
Edit: That's what you get for playing around with stuff BEFORE asking questions.