To add products to a wishlist, go to that product's page, then click the "List This Product" link just below the "Add to Cart" button. When you add an item, you can specify the quantity you want, and add comments.
If you want people to be able to buy things for you from your wishlist, be sure to specify a shipping address. This address is only visible to you—it shows as "So-and-so's Address" on the purchaser's order confirmation page and order history. To preserve privacy, shipments to wishlist addresses are not trackable even if the purchaser selects UPS as a shipping method. If you don't select an address for your wishlist, purchasers will need to deliver those products themselves.
In addition to keeping track of how many you want of a particular product, wishlists also keep track of how many were purchased from paizo.com. If someone buys you something on your wishlist from your friendly local game store, you can adjust the quantity received for that item yourself. If you add an item to your cart from someone's wishlist and that item gets purchased by someone else before you check out, we'll highlight that product in your cart so you know it might not be wanted anymore.
By default, new wishlists are visible to everyone, but you can edit your wishlist to make it private. To make your wishlist easier for others to find, fill in the "full name" field in your profile. (Click your name at the top of the page where it says "Welcome, your name" to get to your profile.) There's a new "wishlists" option in the search box.
You can have more than one wishlist, and you can move items between lists by editing the list. You can also add a description for your wishlist (wishlist descriptions and item comments can include standard message-boards BBCode markup). If you don't want a particular wishlist anymore, edit that list and click the "Delete This List" button.
Each wishlist has an RSS feed so people can be notified when it's updated, and there's an RSS feed for your wishlists page so people can find out when you create a new list.