Replacing a single-pole switch with a dimmer
This is actually how you want to see your switches done. Except for the ground. You should always ground the switch when able and I don't see any reason why this one couldn't have been grounded. One of the wires into the box is the power (it's the wire coming off the switch that leads to the nutted group). The un-nutted black leads to your fixture. There's a matching white (neutral) that's pigtailed as well (you don't need to mess with it).

You should do as the instructions say and use only the screws indicated. The extra screw is for a three-way. Since the dimmer has a ground wire, just add it to the existing ground (bare) pigtail.