Attaching the Deployable Lab, Habitat, Cargo, Assessment Unit

A similar process applies. Place a main part on the ring (Command, Logisitics, etc). Make sure this part is attached to the ring at right angles to the view (not on a 30 degree angle or you will have rotation problems).
Build two docking ports and the unit you want to attach (Assessment Unit, Habitat, Cargo, etc).

Lift the docking ports and unit you want attached off the ring.

Connect them to the underside of the main part (USE ALT to SNAP!).

If you zoom inside the parts (shift - mouse scroll wheel) you should see it connected like this. You can right click the docking ports and "enable staging" if you want them in the staging list.