Groundfloor,
On a 320/30/40 you are the PF taxying the ac and should only have to glance at your ND where you should see TA ONLY in a white box at the bottom. You then reply set.. The PNF will have switched the 3 posn rotary switch to TA/RA as part of his/her pre-takeoff actions.
The code and modes are done at the gate..before engine start, transponder should be in auto mode..
He's referring to the ATC...SET in the before takeoff checklist, ABOVE the line (not below it). You will only get the "TA Only" on the ND when the xpdr is switched to TA or TA/RA. The only way to check if it is set or not (with the correct code and modes) is by looking at it.