If you have purchased a legitimate copy from a third-party retailer, activation is usually straightforward: Launch the game after installation. Click the "Register" or "Unlock" button.
: For the EA App version, deleting the 71592.dlf file from C:\ProgramData\Electronic Arts\EA Services\License often fixes "missing key" bugs. Plants Vs Zombies Registration Key List
The safest and most reliable way to obtain a key is to purchase it from an official or authorized retailer. Below are the best sources: If you have purchased a legitimate copy from