"Single Mission" - (Choose any Character & Name) -
type the unlock code on your keyboard (again CaSe SeNSiTiVe)
The bonus mission will become selectable (no longer grayed-out).
This mission can also be enabled on the Mission Selection sub-screen of
either "New Game" or "Load Game", which is the Campaign Mode where you play
through the missions in order. The "Single Mission" option seems to be the
exact same missions, but it allows you to play them in any order.
No other unlocker will be released by us, since "Single Mission" already
give you access to all the levels.