If I understand the question correctly, then:
Steerpoint or waypoint are simply points in space used for navigation. You have to make something the SPI to get sensors to look in that direction. To make something SPI, you have to point a sensor at it and TMS Up Long. You can point with the TGP, TDC (HUD 'box'), or, in certain instances, the map. To point using the map, you have to be be able to designate with the cursor. You can most easily do this by making a mark point (TMS right short).
Someone please correct me if any of this is wrong.
Edit: If you are just trying to get the TGP to look at a waypoint, cycle to make it the steerpoint, then China Aft Long. You can set SPI to Steerpoint with TMS Aft Long.