Problem activating Product Designer Pro

Getting frustrated trying to activate the Pro version of the software that I purchased. I've found and followed every instruction I could find, in order to get up and running...but still nothing. I've also initiated a couple "Support Tickets" but no response. Really thought this plugin would be the answer to all my problems. But so far, it's just become another one. Where I'm having a problem is finding "where, on the settings page,  am I supposed to copy and paste my license key#, in order to activate the PRO version?" I've looked and tried everything and still can't figure it out. Could you please help me through this process? Thanks