That depends on your problem. Problems can be categorized by error message:
"Incorrect activation code!" - make sure that you enter your name exactly as it appears on your order e-mail (case sensitive).
"Activation code is not valid for current version." - free upgrades are provided for 1 year after purchase, to use the version released later than this, you should purchase an upgrade (costs 50% of the original price). Please contact us to get an upgrade.