From Villa Failte you will be using the yellow and red lines (broadly the same line just opposite directions ). Walk for 15 minutes to Place de Gambetta, which is the square in the centre of Argeles Village where the small Carrefour is located. The stop is just beyond the far corner of the square. One side goes towards the port and beaches (yellow line ... you will see a yellow circle on the sign at the trainbus stop) and the other side is for the returning trainbus (red line ... red circle on the sign). The cost for the journey (wherever you are going) is 2.5 Euros each way. Payment can be made once on the trainbus (directly to "conductor") or in advance by purchasing book of tickets from the kiosks at the Centre Plage.
If going to the Port or beaches from Villa Failte then get the bus from the stop on the right hand side of the road with the yellow circle.
We took the trainbus in the evening when going to the Port for dinner. There are scheduled times for the trainbus stops (see the trainbus schedules available from Tourist Offices, the sales kiosk, online, etc). Broadly speaking during peak season every 20/30 mins and running late into the night.
If leaving the Port to return to Villa Failte the stop is found by turning left when leaving the Port and following the path / road round to the right of the roundabout. The stop (red circle as you will be going on the red line) is about 100 yards further on on the right hand side (there is an Estate Agents / Immobilier just across the road).
Worth doing at least once during your trip ... if only for the experience ! PS, the picture below was taken very early in the season ... in July / August the trainbus is much busier !!!