The Melee Disc is first obtained in the Human-Volterite Alliance war factory from the corpse of a Skull Crusher that accidentally walk into a electricity field. In combat it is very useful especially against enemies that only attack at close range. Melee Discs are also used by Skull Crushers who carry one in each hand. They throw one and use the other to punch with.
It cannot be used in the N.O.V.A 3 campaign
The Melee Disc is classified as a item and can be unlocked at level 28. A good strategy is to sneak up on snipers and attack them.
In N.O.V.A 3, it is unlocked by default and is used when the melee button appears when next to an enemy.