CiteULike can automatically extract the citation details from certain supported sites . If you want to post an article available on one of these sites, you should see the bookmarklet posting page or, if you're still having trouble, read the posting FAQ. It's quite often possible to find your article on one of these supported sites. For instance, biomedical papers are almost always listed on PubMed.
If you use this page to type the details in yourself, your article will appear in your library as normal, but not on CiteULike's front page.