As Axlle mentioned, you need to get at least one of them up to level 19 and do the quest to unlock it. Once that's done, the ability to meld materia onto items is class specific, so you'll be able to meld materia onto the same types of items that you could make or repair. With Blacksmith and Armorer, for instance, you'd be able to attach materia onto metal armor or tools, but not onto cloth or leather armor, wood items, or jewelry.