I am not able to select a drive, the directory is greyed out and when I do install it
You are probably assuming the products would install "into" the sim, which is no longer the case. Even in FSX, we do not install into the sim folder anymore, but in an FSDT folder, which by default it's "C:\Program Files (x86)\Addon Manager", which will be used and shared by all sim and all products.
This folder can be chosen freely only the first time you install an FSDT product, so it's now greyed because you are running another installation (again, regardless of the sim).
So, first, I hope you haven't changed default and set the FSX folder as the destination folder, because now that you finally have the freedom to install OUTSIDE the sim, not accepting the default folder and installing INTO the sim, it's probably the worse possible idea. Of course, we allow you to change the default, because you might not have enough space into the C: drive, but that doesn't mean you should install inside the sim and, in fact, it's best if you don't.
So, if you installed "into" FSX, it's best you uninstall *everything* AND Reply YES to the question "Do you want to remove the Addon Manager".
This will CLEAR your previous choice, allowing to either accept the default location (which it's FINE if you have enough space on C:), or change it to another drive, with the recommendation of NOT using a folder inside a sim, because this location will be shared by FSX and P3D.
I am able to register the product but it stays in trial mode.
This doesn't have anything to do with the previous issue. Please clarify what do you mean with "Stays in Trial mode". First, it might be best if you said what you are referring to with "the product". Which product ? A scenery in Trial mode will work entirely for 5 minutes. GSX in Trial mode will work only at Trial airports or FSDT airport. This is Trial mode.
Instead, if you don't see any buildings, or you don't have GSX starting, that's NOT Trial mode. That's just "Software not running".
So, please, clarify if you are really stuck in Trial mode, or it's just the software that is not running, so you don't see buildings, and/or you don't see the GSX menu.