Вы ищете оператора ИЛИ?См. http://www.w3schools.com/js/js_comparisons.asp
var letter = document.getElementById('letters').value
if (letter == 'man' || letter == 'stevesho.com')
location.href = 'http://www.google.com'
Другой вариант, который может представлять интерес;
var key = document.getElementById('letters').value
var collection = {
"key1" : "url1",
"key2" : "url2",
"key3" : "url3",
"man" : "http://www.google.com",
"stevesho.com" : "http://www.google.com"
};
if (collection[key]) {
var url = collection[key];
alert(key + " = " + url); // DEBUG
//location.href = url;
}
else {
alert("Access denied. Try again.");
}