You can assign guards and workers through the ACTIONS page of the NPC. Darkmere Mill/Farm produces wood, which you can use or sell. You can also assign guards from the Darkmere Mill page -- up at the top, hover over the little icons and you'll get their info popups.
EDIT: Oops, the little icons on the Mill page won't be there unless you also have Fort Sera. The icons send the guards from Fort Sera if you have guards there (or from the other properties if you own them).