In NAV mode it will be at the lower portion of the HUD. But if you choose to display DED info on the HUD it will be above. Lowering landing gear will make the heading tape float above the FPM. In AG mode it should already be on upper side.
Even if the visibility is clear, and you have good visibility and image can be clearly distinguished on MFD, your airframe needs to be at certain range. For example you cannot lock a vessel from 20+nm even if its a huge target on the MFD where real life image processing algorithm would lock it right away.
DCS does not use image processing like real maverick does and rely on magic if the range is okay. For example you can lock a target even if it is behind the cloud and maverick actually points to it.
I reported this before and it was tagged something like provide track or public evidence I can't remember which one. I did not provide because everyone can try this without needing evidence. Try placing it on a big vessel and try locking from 20+nm.
Terzi's post in Magnetic Hdg Tape position in HUD in F-16 was marked as the answer