Trouble with overmelding?

Hopefully someone can answer this question -

I have used a shitload of gil on gathering materia in addition to those I have converted myself. In addition I have red at least two different guides on overmelding to be sure not to waste any materia.

Yesterday I started to meld, but it wont work. The guide tells me to meld in the most expensive materia first, to be sure not to destroy them.

My problem is that when I have melded in the first materia, it does not work to meld in more.

What am I doing wrong?
(I have the catalysts and the lvl50 class needed for the meld)

Question 2 - if I can figure out how too overmeld :-p Which DoL class do you recommend to boost first?

Have you done the quest with the materia goblin in thanalan that grants over melding?

There is a quest for overmelding!!?

I was prepared that there was something obvious I have missed. XD

Many thanks Scotchio!

Yup, the quest should be this one -> http://na.finalfantasyxiv.com/lodest...t/b75b2f62483/