User defined functions in Amibroker

I've created a simple function to determine the percent above or below the 200 day MA. I want to use it in explorations. why doesn't it work? do i have to save it in a special directory? compile it?
function PCT200 ()
{
result = ( Close / MA( Close, 200 ) ) - 1;
return result;
}

What do you mean by "It doesn't work"? Explain what you do.