http://www.gamefaqs.com/portable/ds/code/931475.html
Here it says how to obtain it. And it seems that you have to finish the game to use it.

Did the room you visited was in the bottom floor of area N-1? there is a door there leading to an area wher you must enter the 3rd door and a data stone. Ok and don't hit Omega's weak spot.

I don't think that Omega has a weakspot. Anyway, what you do is you defeat Omega, then you have to go back to the area where you can fight the MMZ3 and MMZ4 bosses. DO NOT FIGHT THEM. Just keep going to the far right of that room then go inside this door, in the door will be a weird rock you pick up... after beating the game you go and have a conversation with the nerdy guy who fixes your biometals and he'll say it's really a biometal then  you can use it.

I had to beat Omega multiple times to do this, because if you run out of lives on your way back then you can't get the metal and must defeat him all over again. I would die on the spike pit on the way there then have to "retry mission" and his metal wouldn't be there when I got to the spot. It's a good idea to stock on extra lives before you go to fight him.

Originally posted by DRAGON-AKUMA666
trust me duo, i tried everything, could it be that i am playing on easy?

I think you can only get Ox on normal and hard, I feel like I read that somewhere. I never played easy, sorry.

Originally posted by LordKaiser
Of course taht was the problem! Who want to reward a weackling that plays the gae on easy?

The game is pretty challenging. After playing through normal I have to say that I do not judge people for playing on easy, it's just too steep a curve to get good at... especially for people new to MMZ.