Method 1:
Edit the file index.php and after line 37 which is global $userpro; add this:
$userpro->validate_license('whatever-license-key-you-want-displayed-here');
No need to do anything else, upon reloading the dashboard it will be activated and nag messages will be gone.
Method 2: Just use below key to activate the plugin
0d9266ab-4233-42ee-b48b-5fc5bfb8ee5f