I had the same problem and I share the solution (not trivial for a newbie).
There is no installation instruction, and the downloadable file in the shop website is only the update to the last version of Padshop, not the update from Padshop to Padshop Pro as we both believed. If you already have the last version, the update will simply not do anything and exit. But they send you the activation code in the email. This has to be put on the usb e-licenser key by use of its software (currently named “eLicenserControlCenter”). Open it, you find “enter activation code” in the menus. Select it, and anter the code by copy/paste from the email.
Then you launch Cubase. The instrument list in Studio/VST plug-in manager is not updated: the name in the list is still Padshop (without Pro). When you insert an instrument track in Cubase and select “Padshop” (without Pro), now Padshop Pro opens and there you go.
I suggest Steinberg to make the installation instruction clear in the shop website.