I'm going to preface this I'm guessing on a fair bit of this, I'm not exactly in tip-top shape critical thinking wise atm, and I don't want to look like an ass in front of the resident forum Harrier Pilot .
I'm assuming the actual attack logic is similar to the Hornet's in that there is a logical object called a designation, which is roughly analogous to the SPI from the F-16/A-10. It can be a waypoint, coordinates, a radar fix, a TGP position, or a position designated directly from the HUD, but it basically is something the Mission Computer recognizes as the thing you would like most to kill at the moment.
If I had to guess your options in the AV-8B would be selection INS coordinates as a target directly, selecting a target utilizing the Angle Rate Bombing System (ARBS), a compilation IRTV unit and LST in the nose, using the TGP, or selecting a target via the HUD.
INS ought to be easy, there's a MFD command to designate a waypoint as a target.
ARBS as you've found out, has two basic modes. The first is the Dual Mode Tracker (DMT), which is a TV that locks on to contrast. Using angle and elevation information from the aircraft it estimates range and can generate a CCRP (AUTO in the McD product line), or CCIP Lead). The second is LST, which is Laser Spot Tracking, or latching onto a spot provided by another source.
TGP is the mode I know the least about, but this should be the most similiar to the A-10 (It's the same pod). My understanding from the lack of mention in this manual, and comments by Zeus, is that it's more or less "after market", with less stream lined integration compared to the other sensors.
Finally I assume there is the ubiquitous ability to select targets directly off the HUD.
Thus ends me talking out my rear.