Simple answer: the same way you cast a spell. The command for using an item is (you guessed it) /use. Like /cast, its simplest form takes the name of the item you want to use:
/use Green MechanostriderThere are also a couple other forms of the /use command:

/use <inventory slot>

This form of use allows you to use an item in the specified slot. See http://www.wowwiki.com/InventorySlotId for a list of the slot numbers. Example:
/use 13Uses whatever is in your top trinket slot.

/use <bag> <slot>

You can also use an item in a specific bag location. Lets say you always keep the food you want to feed your pet in the first slot of your backpack. You can easily write a macro to feed your pet as follows:
/cast Feed Pet
/use 0 1Bags are numbered 0-4 from right to left (0 is always the backpack) and the slots are numbered starting at 1 going left to right, top to bottom (like reading):
1 2 3 4
5 6 7 8
...or
1 2
3 4 5 6
7 8 9 10
...Trading risk of confusion for completeness, I'll let you know that /cast and /use function exactly the same way. /cast can use items and /use can cast spells. This isn't very useful for simple macros like you've seen so far. However, when you start dealing with macro options and sequences you'll be happy to know that you can intermingle items and spells in the same command.