Please be as specific as possible when letting us know what is missing or incorrect. For example, say "this game's trophy difficulty should be 8/10", with a link to a website corroborating this difficulty estimate (if possible), not "this game's trophy difficulty is wrong".
This set contains the following two multiplayer missions: 1) 'Dancing Sands' 2) 'Avian Avenger'
1) will be unlocked once you progress as far as Phase 8, while 2) will be unlocked once you reach Phase 13.
The materials obtained upon clearing the missions can be used to create new armor.
Use the Portal Stone to access the multiplayer lobby and select DLC to receive the additional missions. If you wish to play with multiple players, then each player must own this additional content in order to proceed.