It has been a while since I installed Print Studio Pro. But as I recall, it has its own installation routine. You don't install it using the plug-in manager. I'm using it occasionally with Lightroom CC now, but I have also used it with Lightroom 5. Print Studio Pro runs as a plug-in, but it installs like a regular program.
I tracked down the problem with the aid of very good support from Canon Europe. They were able to tell me that they were very sorry, but they had not yet delivered compatibility with LR6. All my fault really, I had misread the Canon compatibility statement and bypassed the message on installation which told me that a compatible version was not found.
Silver lining is that I learnt a lot as I tried to solve my non-problem.