It should be on the package you purchased. If it is a trial version that came with the MAC, then you have to purchase the product key from Microsoft or a retailer.

You can get the key from Microsoft. Office 2011 has been released so if you are buying a key you should get it for the most current Office product as long as your computer will suppoet it. You can find out all you need to know either way you want to go here (http://www.microsoft.com/mac/).