There should be a small search box at the side of the screen, marked search, followed by Go and Search. Just type what you're looking for into the search box and press the Enter key. Or, click Go or Search. The difference between Go (same as enter) and Search is that Go first looks for an article with the given string as its title, and if one is found immediately goes to the indicated article. If Go fails to find an article, it then automatically executes Search which is a full-text search for the given words.
Wikipedia's default search mode will turn up results with any of the words in your query. For instance, search engine turns up many results containing only "search" but not "engine" or only "engine" but not "search" in addition to the ones you probably wanted, which contain both words.
To limit to results that include all words, put a "+" at the beginning of each word: +search +engine returns only pages containing both words, like Google's default mode.
You can also do a phrase search by enclosing words in quotes: "search engine" turns up a smaller set of results, which not only have both words but have them in order.
To exclude results that include some word, put a "-" at the beginning: search -engine
Avoid short and common words
This is the most likely cause of an unexpected failed search. If your search terms include a common "stop word" (such as "the", "one", "your", "more", "right", "while", "when", "who", "which", "such", "every", "about", "onto"), then your search will fail without any results. Short numbers, and words that appear in half of all pages, will also not be found. In this case, drop those words and rerun the search.