how are people managing to use manual chain attack zzz?

The bit people miss is the order. Whoever you pick last is who ends up on field when the chain finishes, so plan it so your main damage dealer is the one left holding the enemy. And you don't have to take the chain at all, if you let the timer run out you just stay on your current agent.

you need the enemy stunned first. fill the daze bar, then land a heavy hit on it, EX special or a heavy attack finisher works, and the chain prompt pops up. you've only got a couple of seconds to press left or right to pick who comes in. if it never shows up, you're probably hitting it after the stun has already started wearing off.