Hi Horace ,
With the latest version of Buttonizer, version 3.0, you no longer need the pro version of the plugin.
If you have already installed the 3.0 plugin and connected your site, you simply have to go to the Buttonizer dashboard here:
You can then select "Manage site".
and scroll down to "Change license".
You should be able to see your license in the dropdown. After selecting it, you should now be able to use the pro features for your site! 😃
Let me know if you were able to activate the pro version.