If the nets are already part of a coupling group, then you can't use the right mouse option. You have to go into the Edit Transmission line dialog and move the transmission lines so they are part of the same coupling group on the 'Move to Coupling Region' tab. You can easily tell if the transmission line is part of a coupling group in the schematic because the left or right side of the transmission line symbol will have a green dot on it.
Another option would be to remove the transmission line from a coupling group by selecting the 'Uncoupled (single line)' Stackup button (instead of selecting the Coupling Stackup button). This would allow you to do the right-mouse click couple option.
Thanks Steve, want I learned which is to obvious you select your T-lines, at least two then right mouse select and you get the 'coupled' option. This what the user guide was basically telling me but the simple thought process was not coming through. Need to start using HyperLynx more often than ever 4 years.