...is inserted into the front of the Garmin unit.
The threaded fastener in the Txponder has a socket head that accepts the allen tool. This fastener is threaded into a cam flipper that is internal to the front of the Txponder unit.
You should start the installation by first using the tool and turning the tool / screw in a CCW direction. This will swivel the "cam flipper" and position it in the proper position / orientation for the install. Once the flipper is fully retracted, insert the unit in the tray, and while lightly pushing the unit in...use the allen tool and turn CW until you feel resistance against the turning.
Turning in the CW direction caused the "flipper" to flip, rotate into the slot on the front of the txponder tray, and will "cam" (pull) the unit into the tray as the allen tool is turned.