In order to track a TACAN in the Hawk you'll need to do the following:
On the TACAN panel, tune the TACAN frequency (for example, Wittering 123X)
Set the TACAN mode to TX/RX
On the main instrument panel, the ILS/TACAN button should be depressed until TACAN is illuminated in green.
Once the aircraft is in range of the TACAN, the distance and bearing will be indicated on the HSI.
If you wish to use the Autopilot controls on the EFB to track the TACAN, you'll need to switch the UHF Ae switch to "Lower" on the far right side of the right console (this toggles between traditional navigation and GPS navigation).
Hope that helps.
Mark - Just Flight