Hi and welcome to BookCrossing! To put a book on your wishlist, go to your profile. Then, you will see something that says "All Books". Click on it, and you will see a filter called "Wishlist". Click on that, and you will find your wishlist!
Hi! The global-overview pages all have an "add to wishlist" button on them, so if you can find BC copies of the books you want, click on the "global overview" link or icon and then on the "add to wishlist" button.
Once you have a wishlist, you can add wishes directly, even for books that aren't listed on BC yet. To do that, view your wishlist from the listbox in the upper right corner of your profile page as Readermax described, click on the "add new title" button, and fill in the fields. [That button doesn't appear unless there's at least one wish on the list already, so you will have to start with the global-overview method.]
Hope that helps! But do note that BookCrossing is mainly about setting your own books free; some BCers do like to try and fill the wishes of others, but it's not the main purpose of the site. If you're interested in trading books by mail, check out the Random Acts of Kindess forum ( http://www.bookcrossing.com/---/10 ).