Hello, I have Lexicon reverb working in Studio One, make sure the default install location is your vstplugin folders, one for 32 bit and the other 64 bit. Add these locations to folders to be scanned for vst plugins in Studio One's options.
Alternatively, add the default install locations (Program Files\Lexicon\MPX Native Reverb and Program Files(X86)\Lexicon\MPX Native Reverb) to the folders to be scanned for vst plugins. Assuming you're using Windows.
Hope this helps.