acquiring henchmen

I have a couple characters in nightfall that have been there awhile how do I get olais and zenmai? One has Olias but I have long past both quests with these characters and just brought a lesser played character to nightfall and got both

Get the quest from consulate docks, and do the quests in the appropriate campaigns.

Here's some links which should assist you:

http://gw.gamewikis.org/wiki/Zenmai
http://gw.gamewikis.org/wiki/Olias

By the way, they're Heroes, not Henchmen.