"Cannot find a library instance of <library name>." means that the library file is missing. I downloaded your file, and you aren't actually including a copy of Dewdrop-2.0... or copies of Tablet-2.0 or FuBarPlugin-2.0 for that matter.
You could update your addon... but doing a proper update at this stage would entail a complete rewrite of the addon, as all of the libraries the current version uses are deprecated, and no longer supported or recommended for use.
Or, you could just use an existing addon that shows you drop locations in tooltips. Here are a few: