plate is one of the default plugins
it is placed in your LADSPA folder and named 'plate_1423.dll'
Look for it there. If you dont have that dll, something was missing in your download
Did you get that from the official site?
https://lmms.io/download/#windows
AND
are you using an official release or a pre-release -only 1.1.3 is guaranteed for production, the 1.1.3zxy is not