You have to progress the story far enough to get your retainer. After that, make your retainer and you give the items to him to sell for you. When you get the retainer there is a tutorial on how to use them.
And I believe the point in the story is just before Bowl of Embers. --- PSN: Ack-Tar FF 14 Behemoth Server
Its a level 17 or 18 quest that gives you access. You get access as soon as you start the investigation that leads to Ifrit, not afterwards.
Its really not miss-able if you are reading. They tell you now you can hire them as a reward for doing something. Its not an actual quest reward(stated that way for min spoiler) --- psn id: dmljr xbox360 id : Sword of Dawn Wii: 0186 9165 8762 2178