You can't use the PTT jack on the FT-840. That's used to externally key the rig, not for the rig to key an amp. You'll need to go to the Band Data connector on the rig. Pin 2 on the rig end goes to terminal "RY" on the 2100, and Pin 8 on the rig end goes to terminal "E" on the 2100. The 2100 uses a 12V relay for keying, so you don't need any type of additional interface.
That's not quite right. Pin 2 of the 8-pin BAND DATA plug goes to "RY" on the FL-2100B and pin 3 of the BAND DATA plug (ground) goes to "E" on the FL-2100B. Pin 8 is for TX ENABLE when controlled by a QSK amplifier, which the FL-2100B is not. So for use with the FL-2100B, you don't use pin 8 of the BAND DATA plug for anything.