The .artcon file is the database that comes with Art Conductor 9 and later, and is required for the integration between PlugSearch and MetaGrid.
If you are not using MetaGrid, disable 'Connect to MetaGrid Pro' in the preferences of PlugSearch, and PlugSearch will not show the warning. If you are using MetaGrid for articulation selection, make sure the .artcon file is located inside the ' Art Conductor Logic' folder.