Use the Soundex search. Names are often misspelled in historical records. Soundex allows you to search for last names that "sound like" the one you're looking for. Try this even if you think the spelling is obvious.
In the KEYWORD box place the county name, if you know it, to reduce the number unrelated persons. Be sure the county spelling is correct or it won't return any results. If the county is not know then leave blank.