Triplexer goes on the back of the receiver. Plug the three lines into the Sat1 port, Sat2 port, and Home Distribution ports. The cables should be labeled and the triplexor says which Sat port is which.
Then, you put a diplexor anywhere in between the ground block and that wall plate (i.e. somewhere along the line which is most convienent to connect up the second TV coax). If you are wanting to add an OTA antenna in the mix, you will need to split the Home Distro (yellow) cable on the triplexor to go to both the home distribution output and the OTA antenna input (black connector).
On the diplexer side, you will see a In/Out port, Sat Port, and UHF/VHF port. The IN/OUT goes on the receiver side of the line, the Sat goes on the ground block side, and the TV2 coax goes on the UHF/VHF side (or the OTA antenna gets plugged in on this).
This page has some examples of how to hook up diplexer. Just think of a triplexer as a diplexer and a separator built into one piece.
How to Extend UHF / UHF Pro Remote Range