A Regional Jet First Officer reported not arming the approach during a visual approach to Runway 10 at ATL resulting in an overshoot and a TCAS TA. The Captain detected the deviation and took control to return to the localizer. Fatigue was cited as a contributing factor.

Narrative

[We were] cleared for the visual approach near PENCL Intersection for Runway 10 at ATL. The autopilot was on and we were flying a heading of 070 degrees to join the final approach coarse and descending to 4;000 FT. Sky was clear and visibility unrestricted. Prior to localizer coming alive; we were slightly high on the glideslope. I reset the altitude alerter and selected vertical speed on the flight control panel in order to keep our descent going to capture the glideslope. In the meantime; the localizer started to center up. With the preoccupation of the glideslope; I either did not select APPR mode; or did not press the button hard enough to arm the APPR mode. We flew through the localizer; at which point the Captain stated that we were not armed for the approach. He disconnected the autopilot and turned the aircraft back to join the final approach course. We received a Traffic Alert as we were turning back to the localizer. The Approach Controller then advised a turn to the localizer after we had initiated our turn and pointed out traffic on the parallel Runway; 9R. We intercepted the LOC and landed with no further incident. This was the last leg of a five leg day at the end of a three day trip. I believe the combination of everything happening at once; with the glideslope; localizer; resetting the altitude alerter and Vertical speed caused just enough of a distraction to lead to us not double checking to make sure the APPR mode was selected on the Flight Control Panel. Sometimes complacency combined with a little fatigue at just the right (or wrong) time can create a serious situation in just a matter of seconds.