WIth Winamp Pro for Windows you will be prompted to enter your Registration Key at the end of the installation process.
If you miss or cancel that dialog, you can always manually enter the key later.
Go to: Winamp > Preferences (Ctrl+P) > Winamp Pro
Under "Registration Information", copy+paste your exact Registration Name into the top field,
and the exact Registration Key into the bottom field.
The name is the same as the one you used on the registration form
and the same as the one the confirmation email is addressed to (Dear "Your Name").
Click "Submit Registration Key"
Say "Yes" to any Windows UAC prompt, if enabled (Vista/Win7/Win8-only).
It should now say "Valid 5.0 registration key" and the Pro features will be unlocked.
If it says "Invalid Registration Key", and your Windows OS is Vista, Win7 or Win8,
then close Winamp,
and reopen it by right-clicking the Winamp desktop icon (or winamp.exe itself)
and select "Run as administrator".
You should now be able to follow the above instructions again to enter the registration name & key
and hopefully this time it will be accepted.
Now close and reopen Winamp in normal user mode for standard everyday usage.
Q: Is there a registration key for Winamp for Android Pro?
A: No. Orders on Google Play are tied to your Google account. Pro features are automatically unlocked.