To play the BFG Edition of Doom3 on OpenBSD you need
the original game.  And in order to get the game data
it must first be installed with the Windows version of Steam.
Copy the game data from
C:\Program Files (x86)\Steam\SteamApps\common\Doom 3 BFG Edition\base
to ${TRUEPREFIX}/share/RBDoom3BFG/base/ change directory to
${TRUEPREFIX}/share/RBDoom3BFG and run 'RBDoom3BFG'.

To play the original 2004 release of Doom3 see games/dhewm3,
this port is only for the 2012 'BFG Edition' release.