Alder is at the bar in menasa so go to the bar before you go to the hells wall fight and you talk to him in the bar he says he will join you if you beat him in the arena but don't worry if you lose you can fight him over as many times as you like and thats the way to get alder!
New Wanzers
To unlock a new Wanzer for both the OCU and USN, beat the corresponding storylines once through.