Got IT!

I have the same issue.  If you figure it out, please post!  I can't manage to get Melodyne to show up in Platinum, but it still works fine in X3.

Most common reason for not seeing Melodyne as a region FX is that you have not installed the VST3 version.
Or that "C:\Program Files\Common Files\VST3" is not in your VST scan path
Or that you installed Melodyne to some other location that is not in the scan path

I resolved the issue on my end.
I decided to check "C:\Program Files\Common Files\VST3" to make sure that Melodyne was there.
It was, but it was in a folder called Celemony which had a folder called Melodyne which contained the dll or whatever type file Melodyne is.
I took the dll out of the two folders and brought that up to C:\Program Files\Common Files\VST3 and now it is back in X3 and is visble in Platinum.

I figured out in the command center was dictated where my vst were installed (via the little gear icon). I had to change that. I thought it would remember from X3.  Anyway, all is well now.