I can't see anything obviously wrong with your ISBoxer configuration. The cause may lie elsewhere.

Some things to try:
- Make sure you've closed all game instances and re-launched your Character Set.
- Make sure you test this on a vendor, not with hostile NPCs in combat. Some classes/specs have non-obvious IWT behaviour in combat.
- Ensure the ISBoxer addon is enabled on all characters.
- The Interact with Target Mapped Key (in ISB42 - Base Hotkeys) should also make all your slaves assist your main. Is that happening?
- Temporarily change the Hotkey for the Interact with Target Mapped Key to Ctrl+X or Shift+X so you can test whether the in-game IWT keybinding (X) works. You'll need to make sure you have a target and are in interaction range when pressing X.
- Confirm assist and follow work as expected.

I must admit I'm a bit stumped.