Adding an existing item on your wish list should be very easy to do. You can add something by clicking on the Add item link. At the bare minimum all you need is the name of the gift and how many of them you want (which defaults to 1).
You can then, if you want to, add a brief description of the item. This is often useful to do if the page you're about to link to has many different items on the same page or if the item is only available at a bricks and mortar store. You can also change the rating from the default of 5 to indicate how much you want this particular gift. The default sort option lists items in order of this rating.
Possibly the most important thing to add other than the name are links to where you can buy the item online. It's best to link directly to a page where the gift can be bought to make it easier for people to find the item. You also need to put a price in too.
Lastly you can define where and who can see the item. Tick at least one of your wish lists - it'll default to either your first wish list or the list you were on when you added the item. You can also put in a list of tags that help group similar items together. Then set whether the item is public or limited to a particular group of users. This can include one of the groups you've set up made out of your friends. Items inherit the visibility from the wish list they're on too, so for someone to be able to see an item they need to be able to see the item and the list. If an item isn't set to Public viewing, it cannot be shared on your Facebook profile.
Most of the above also applies to editing an item too. You can edit an item by clicking on the gift's name when viewing one of your wish lists.