When it is your turn during the battle, click the Crown Shop button and navigate to the Henchman tab, then choose your henchmen. I personally find it more useful to switch to list view when purchasing henchmen, and we are looking in to ways to make it much easier (and faster!) to purchase hired help.
I cant get through Estrakir Gloomthorn with henchman. I need help. Does anybody know a link for them anymore??
As Prof. Greyrose mentioned, the henchmen can be bought through the Crowns Shop while you are in battle.
But if you are looking for mercenaries, you might want to check out the fansites. A good site is Wizard101Central.com and you can do a "search" on that site.
