Do you mean that you are looking at a listing and you would like to add it to your Wish List? That's easy, -- about a half inch below the 'Bid' or 'Buy it Now' button is the phrase "Add to Watch list". If you prefer to add it to your Wish List, click the arrow to the right and select Add to Wish list from the drop-down menu. Then you can view items in your Wish List at any time from the My eBay at the top of this or any page.
Is that what you meant?