I have followed the instructions in page 25 of module 8 in the W5 Exercise and reference guide to create driven dimensions to avoid the need for students to do math to ascertain dimensions they need when building the product they have modelled.
In Dimension - New references, I selected an edge and it changed to red colour but no dimension resulted.
What have I overlooked?
You need to place the dimension using the middle mouse button. For example, if you were dimensioning the length of a block, you would left-click to select both ends of the block, then you would middle-click to place the dimension value.
Note this is the same process used to create dimensions in Sketcher mode. I would suggest that you teach your students to show the dimensions used to create their model rather than create new driven dimensions in the drawing.