The FT 3120 chips already come with the firmware in ROM and the default comm parameters are TP/FT-10 with a 10MHz external clock/crystal. You can either program your application over the FT network (using NodeLoad or NodeUtil), or you can pre-program the FT 3120 chips using a BP or Hi-Lo PROM programmer and the appropriate socket adapter.
The old Echelon 3120 Programmer is obsolete, and wouldn't work with FT 3120 chips anyway. Hope that helps. You might also check the Echelon Knowledge Database (KDB) -- I think this may be a FAQ in there as well.