When you buy Mailbird you will receive an email with your license key in it. Once you have that, this is what you need to do to activate your Pro account.
1. Go to settings
2. Go to the About tab and click "Activate Mailbird Pro"
3. Paste your license key on the given box, hit continue and your Mailbird Pro will be activated.
If your Mailbird Pro is still not activated, please contact us through firstname.lastname@example.org