To add a ring constraint to the diagram, you should be able to just drag it on from the toolbox and hook it up to the appropriate roles using the same process through which other external constaints are hooked up to roles.
Once it is on the model, you should be able to change what type of ring constraint it is through the property browser (until you've specified the ring constraint type, the ring constraint will have an error on it, so you should be able to just double click on it and have it bring you to the appropriate entry in the property browser).