Un-blocking ads

[The Dude]

I run Adblock Plus for Firefox. A chess site that I have free use on just sent me a message that if I don't unblock their ads or buy a premium membership, they're going to stop letting me play.

How can I tell Adblock Plus to allow ads on just that site? I looked to see if there was a specific "rule" I could delete in Adblock Plus, but nowhere mentioned was this site, so I don't know how to proceed.

Thanks in advance,
Dude