Dash Board add High Sierra installed?

Create a Smart Group with that criteria. There is a checkbox that says "Show in Dashboard" near the top right of the smart group editing window. Check it :)

Create a smart group with a criteria to check OS. Make it look for os "like" 10.13. This way if there any updates, like 10.13.1, it'll still detect it. Then click done to finish editing the smart group and you'll see a "Show in dashboard" button at the top. Click it on and enjoy.