Both quests are in Central Thanalan, an area called Bonfire, to the North of Black Brush Station.
War/Magic Disciple at or above 19 for making Materia.
Crafting (not cooking) at or above 19 for putting Materia into equipment.
The main story sends you there around level 18-20, and tells you about Materia.
Your crafting profession sends you there at level 20, telling you to learn to use Materia.
It's hard to miss either.
Edited, Sep 1st 2013 6:33pm by TirithRR
This. Minfilia (AKA Pinkboobs) sends you there to get a dagger materia-melded (before the Ifrit fight iirc) - this unlocks converting spiritbound gear to materia. Your DoH-class trainer sends you to the same spot for your level 20 class quest - but you can go there yourself @ 19, as they are 2 different quests (class quest has you speak to the goblin, 19 quest starts with the hyur female). The 19 quest unlocks materia melding for all DoH classes
- provided you have sufficient level, of course.