Using Rhino 6 Educational license to activate Rhino 5?

Found a plugin that only works for R5, but I got the full edition of R6. I read that having an R6 license also works to license R5 - is this correct?

Not entirely, no. A V6 license will not work for V5. However, if you bought a full version of V6 (not an upgrade) you can get a free ‘legacy’ V5 license using your V6 key. Follow the instructions here:

Found it. I had t use the serial number to get the license key to Rhino 6, then use that new license key to claim a Rhino 5 copy. A bit of a roundabout process, but, I’m glad it worked out. Thanks!