After you've purchased Social Pro, you'll want to input your license key into the plugin. Here's how you do that:

You can find your Social Pro license key by logging into the Mediavine Marketplace > Account > Account Details > Licenses:

1 - Head to WordPress > Grow > Settings > General Settings > Register Version (all the way at the bottom):

2 - Input the license key you received with your purchase of Social Pro.

3 - Click Save Changes and you're all set!