FPrime is a small (approximately 500K) file that does not even need an installer. After you download it, you should unpack it into the single file fprime.p and then place that file into LightWave's Plugins directory. (You can pick any subdirectory you like.)

Quit LightWave Layout, Modeler, and the Hub. Start a fresh Layout, and use the LightWave "Add Plugin" command, found under the Layout / Plugins / Add Plugins menu. Choose the fprime.p file. LightWave should say either "7 plugins added" or "8 plugins added" depending on your version of LightWave. Quit Layout to make sure your changes are recorded. We do not recommend using LightWave's "Scan Directory" command which sometimes fails and can create spurious errors.

It is very useful and strongly recommended that you create a button to launch FPrime in Layout using LightWave's configurable interface. You can enter this in Layout via the Layout / Interface / Edit Menu Layout command. You will likely want the "FPrime" Generic plugin assigned to a button. You may also want the "FPrime Render" command as a button.