Is my product key out of date?

I purchased Office for Mac 2011 from Best Buy about 18 months ago.

I installed and used it, except I chose to NOT use the Outlook, opting to use Mac's iMail instead.

I've now decided to move to Outlook, and cannot get it to work.

I typed in the product key from the original package, which worked fine when I first used it.

But now I get this error message:

The product key is not valid. Make sure that you entered the correct key. If the key doesn't work, contact Microsoft Customer Service and Support.

How can I fix this?

Thanks,

Jason Beighley

Question about Outlook Express