Here is my guide thing I did on my clan website for the tracer dart. I only found out how to use it the other night, it's great fun.
Using the tracer dart:
This only works with certain rocket launchers, I am not sure exactly which ones currently accept definetely the Carl Gustav engineers 2nd launcher
1) Take ya tracer dart pistol and fire it at your intended target. This can be a vehicle, fixed emplacement, building, person, whatever
2) If you get a successful hit you will get 30 'tracer dart planted' points
3) Whip out your rocket launcher of choice and go to the 'zoomed in' target reticule mode of the weapon ( i don't think you can lock to tracer darts with 'hip fire' mode)
4) You will see a 'broken' square on the darted target. Keep aiming at this until the square goes solid and you get a distance to target indicator by the solid square (eg: 58m). (I also don't know if you have to have line of site to lock on to the tracer dart target 'square')
5) Once the lock on is complete fire off your rocket. You can fire from anywhere, to anywhere and the rocket should still hit the target. Eg: once locked on, bugger off to the other side of the map, fire straight up in the air and your rocket will go really high then fly straight down on to the top of the target. If stuff eg buildings get in the way of the rocket, it will hit these first.
Thats pretty much it
On a side note I believe the 3rd engineer kit rocket launcher unlock locks on to heat sources rather than tracer darts. It actually tells you in the in-game weapons info if a launcher will work with tracer dart.