It is not clear for me which factions would be able to recruit those troops.
In case of cretan archers, they are assigned to greek_cities and macedon (for battles), so if you don't change export_descr_units.txt this unit will be available only for those factions (sorry if this was obvious).

The hidden resource must be placed in the corresponding region (descr_regions.txt) and at the beginning of export_descr_buildings.txt.
Finally the unit must appear in this way:

Code:
        archery_range requires factions { spain, scythia, germans, gauls, dacia, ct_carthage, eastern, parthia, egyptian, greek, roman, } 
        {
            capability
            {
                recruit "merc cretan archers"  0  requires factions { greek_cities, } and hidden_resource kreta
I hope this help.