Validation Script Help

Just copy the entire code, including the last line I provided, under that field's custom calculation script and it will work.

You don't need to modify it at all. Just copy it to a doc-level script, and then call it like this from the custom calculation script of your field:

calcAverage(["fill0", "fill1", "fill2", "fill3", "fill4", "fill5"], false, false);

I've actually written a function that does exactly that, and it's better than the built-in version because you can tell it to ignore empty fields and/or zero values.

/*
    @params
        aFields : an array of the field names to use for the calcluation
        bIgnoreBlanks : a boolean that tells the function whether or not to ignore blank fields
        bIgnoreZeros : a boolean that tells the function whether or not to ignore zero values
*/
function calcAverage(aFields, bIgnoreBlanks, bIgnoreZeros) {
    var total = 0;
    var n = 0;
    for (var i in fields) {
        var f = this.getField(fields);
        if (f==null) {
            console.println("Error! Can't locate a field called: " + fields);
            continue;
        }
        if (f.valueAsString=="" && ignoreBlanks) continue;
        var v = Number(f.valueAsString);
        if (isNaN(v)) continue;
        if (v==0 && ignoreZeros) continue;
        total+=v;
        n++;
    }
    if (n==0) event.value = "";
    else event.value = total/n;
}

Where did you add the script?