The hidden resource is in place, but for whatever reason the unit was not made recruitable(except for the slave faction). Making it recruitable would mean editing the file export_descr_buildings(EDB) from Rome - Total War/data or if you have the mod switch versions, from Rome - Total War/RTRPE/data
Make backups of the file and of your saved games before giving it a try.
If you only want to recruit the unit with the greek cities you only need to add two lines. One under each of the practice_field and siege_engineer sections. To make it easier for you just search this line:
Code:
recruit "greek archer" 0 requires factions { greek_cities, } and building_present_min_level barracks_greek_cities muster_field
and under it add this line:
Code:
recruit "merc cretan archers" 0 requires factions { greek_cities, } and hidden_resource unit_crete and building_present_min_level barracks_greek_cities auxilia_phase3
Repeat this for the second building and then save the file. The unit will be available in Crete/Kydonia and would require an auxilia_phase3 or up to be build in addition to the missile buildings as a precondition. It won`t be available in the other cretan region because that lacks the unit_crete hidden resource. To change that would also probably require to restart your campaign.