You received an email up on purchase of the product/license this contains the key and the instruction steps also, please beware that if you have an existing license DO NOT activate before or on the day of expiry else you will lose the days from your previous license!
Assuming you don't have HitmanPro installed, else continue from step 3.
1. download hitmanpro : http://get.hitmanpro.com
2. let it scan once
3. restart HitmanPro via Windows Start Menu
4. click 'Settings' button in HitmanPro
5. Go to License tab and insert your product key there. Click Activate.
6. If it's still showing an error go back to settings.
7. Switch to the Proxy tab and select "No Proxy"
8. Try to activate again.