You need to hold certain region and build an auxiliary barrck in that region, Can't remember which region exactly but if I remember correctly, its where the Greeks are for auxiliary creten archers, and I believe other region will give you auxiliary archers too, go into missile infantry in the encyclopaedia and go into the specific auxiliary archer and the region should be on the right hand side in a box.
This, I recruited Egyptian archers just fine when I build an Auxiliary Barracks in Lybia.