You need Seeds of Destruction expansion. If it says you own 15 (or more) of 18 expansions on the server select screen, then you meet that requirement. Next, you can only have one mercenary at a time, unless you buy with station cash (or other means) extra mercenary slots. If you already have a mercenary and want a different type, you have to dismiss it first (use the Dismiss button on the mercenary window).
Next, you need at least the amount of cash to purchase the mercenary, as well as enough to maintain the upkeep which renews every 15 minutes. Finally, you have to be a gold member to get / use a Journeyman level merc, otherwise you are limited to only using an Apprentice merc.
You can find the mercenary window by Alt+W to toggle the Window selector window, and clicking on the far right icon to toggle the Mercenary window.
See http://eq.zam.com/wiki/EQ:Mercenaries for more details.
Yther Ore.
Edited, Mar 19th 2012 1:48am by Yther