How to actually use materia?

[MistakeNot]

That's odd, I've gotten it without being a crafter. It's a storyline unlock from what I remember. But while it gave you the meld option, you needed to be 19 in crafting to unlock something else. Before you got meld, your in-game character didn't know about melding so it would make sense to not have the request meld option and I've noted many players in my fc asking where the button was before they got meld. Once they got it, they found they were able to request the melding. I'll have to go back and double check this though.

Nope, you need level 19 in any crafting class (except CUL) before you can do the quest that allows you to meld materia.
For doing the actual melding you also need to be high enough level in the right crafting class (depending on what gear you are trying to meld)

There is a level 19 storyline quest which sends you to Mutamix at the Bonfire, but that quest by itself does not unlock anything.
There is however a level 19 sidequest you can pick up while at the Bonfire where you learn how to convert spiritbonded gear into materia. (This quest can be done at any point after you have reached level 19 - you do not need to wait until the MSQ sends you there.)