You probably have not selected which folder Project 5 should search in. Project 5 has user-defineable, dedicated folders where it searches for all VST plugins. If a valid (and compatible) VST plugin resides in one of these folders, then Project 5 will recognize it and make it available for use. If a plugin does NOT reside in these folders, or is incompatible, then Project 5 will not allow it to be used.

To set up the VST plugins folder in Project 5, go to Options > VST Plug-Ins. You will see a list called "VST Scan Folder(s)." You have the ability to add new search folders by clicking on the "Add..." button. At this point, you have 2 options. You can add the plugin's .dll file to a folder already in the list, or you can add the folder where the .dll file resides to the list. Either way, the next time you launch Project 5, it will scan these folders for plugins, and provided they are compatible, allow you to use them in Project 5.