Returning index of nth occurrence of a substring
There's no need to split the original string or slice away at the string, you can just use the built in indexOf, keeping track of how many times you've looked for the substring. As a personal preference, I'd also use smaller, not as bombastic, variables names.

Also, there's absolutely no need to include jQuery in this - the $(document).ready is redundant.

function GetSubstringIndex(str, substring, n) {
    var times = 0, index = null;

    while (times &lt; n &amp;&amp; index !== -1) {
        index = str.indexOf(substring, index+1);
        times++;
    }

    return index;
}