Most of Ancestry's naturalization records are found under the heading Immigration and Emigration. You can access these records by the following methods:
1) Ancestry Database Card Catalog--type the word naturalization in the Database Title Box. This will give you a listing of naturalization records.
2) Immigration & Emigration--click on the Search tab at the top of the page, then scroll down to Immigration & Emigration and click on that link. Search by first and last name. Note that the search results include naturalization and many other immigration records.