It's important to make sure you are on the right screen for what you want to be doing.
If you want to see who has cited a published book or article, you want to look under Cited Reference Searches. Looking at footnotes, you can follow research backwards in time to see what sources your article used. Cited reference searching allows you to take research forwards by seeing who else cited the article you are using. You can look at the other citations those articles used and build a web of citations and sources useful for your research.
If you want to find (among many other things!) a specific article, articles written by a specific author, or works cited in a specific article, you will want to explore the General Search (this is the search page that comes up when you first arrive in Web of Knowledge.)
