My new shower pan from Tile Ready has a 2-3/8" hole in the bottom, about 1" deep for connection to the existing PVC (black plastic) drain system. The o.d. of the present drain is also 2-3/8" and will, of course, fit into the new pan drain. My question is: how is this connection supposed to be sealed, if at all? Glue? Gasket? Do I have to glue the drain and lower the pan into the mortar simultaneously?

Your 2" plastic drain pipe sticks up insidee that 1" sump area. A ribbed rubber collar is greased with liquid soap and slide over the pipe into the sump area making a seal. If your pan did not supply the rubber collar you can buy it at most plumbing supply or home centers.