Yep, call your CC company & ask them to cancel the transaction. You were scammed. I don't know what version of LW you bought, but Pro costs $18.88 USD for 6 months of updates & email support. LW basic is free.
The only official site to download a clean version of LW is www.limewire.com

You'd need to talk to Mastercard & tell them that you gave the card's security code...they'll know whether you should cancel the card or not. The sooner you report this, the better.

Have a look through this thread at some stage, it's got lots of info about membership & scams:
http://www.gnutellaforums.com/showthread.php?t=28751

It might be a good idea to get rid of the scam copy of LW, for all you know it could have some ad/spyware attached or it might be an old version (the current LW version's 4.12.6). Get yourself a clean copy from the official site:wink:. Start off with the free version & then if you like it, think about upgrading to Pro later on!