If it is a fresh install of Prepar3d v3 with no other versions or Prepar3D or FSX, then you need to install the following:
Program Files (x86) (or wherever you have installed Prepar3D v3)\Lockheed Martin\Prepar3D v3\redist\Interface\FSX-SP2-XPACK\retail\lib\SimConnect.msi
Once installed, TrackIR 5 should work.