The only secure way to obtain a legitimate activation key is through the official channels. Avoid "crack" sites or key generators, as these often contain malware and won't work with XG's server-side authentication.
You can purchase a license directly from extremegammon.com for $59.95 . Extreme Gammon Activation Key
In the registration window that appears, copy and paste your activation code exactly as received. The only secure way to obtain a legitimate