Labor preferences not showing in Dwarf Fortress?

When you select a dwarf in the (u)nit list, you must use zoom-cre(c). This will center the screen on the dwarf in question, and allow you to the it's status(z), (w)ounds, his (i)ventory and, what you're searching for, his (p)references. There, you can change his work preference.

You can also view this screen by pressing (v) in the main screen, and move the cursor to the dwarf you want to view.

@Jupotter's answer is totally correct, but a MUCH better idea is to get Dwarf Therapist, especially if you have a bunch of dwarves to deal with. 2-3 isn't too bad w/ the ingame interface, but once you have 100+ dwarves managing the labor for all of them is much easier with DT.