Place the headtracker.dll as follows
DCS World\bin\headtracker.dll
as well as (yes, indeed, the same file on two locations)
DCS World\bin\headtracker\headtracker.dll
The first one makes Free Track show up in the ingame configuration, the 2nd makes it work in the pit. Factory resets/updates of DCSW tend to erase both entries so double check that.
Don't use trackIRfixers, workarounds, PPjoy and whatnot. Just use the Eagle Dynamics headtracker.dll. Every other method is either clumsy or pre-1.2.5, especially those that tell you to change lines and change settings per aircraft.
Headtracker.dll for x64 installations of DCS World, works on 1.2.5 and 1.2.6:
http://forums.eagle.ru/attachment.php?attachmentid=65358&d=1336086531