for the Underwater materia you need to go down the lift in Junon (where you first go to get the Sub) and you have to get into fights with a monster called Ghost Ship, and with Contain materia I think you have to wait till Mideel has been destroid and then you go there and talk to the white chocobo and scratch behind his ears (also I think), and as for the Choc/Mog summon materia you need to go to the Chocobo Ranch before Meteor is summoned and the chocobo will say wark? and you reply Waark and they will do a dance, you only have to talk to them once to get it.