(for 3.3.5a)
If you want to use any item subclass, you need to have:
1. learned spell which adds items skill (for example 750 - Plate Armor, world.playercreateinfo_spell.db for older revisions)
2. known skill (293 for our example witch Plate, world.playercreateinfo_skills.db for newe revisions)
3. skill "enabled" for your class (again that SkillRaceClassInfo.dbc which is ALWAYS forgot by EVERYONE... *sigh*)