Writing String Value To A Separate Iframe Using Onmouseclick

I am currently trying to create a simple form based web-page (uploaded here [URL] that creates a query for a WMS (Web Map Service).

The page consists of about 10 different forms, which are all put together in a (string) variable named "compiledQuery" in a function named "compileRequest" which is activated via a button. I would like to print out this variable on the page so that the users can see the query for themselves. After doing some research I came to the conclusion that the best way to do this would be via an iframe (I do not want to use a pop-up). The function I use to do this is:

function printRequest()
{
var query_frame = document.getElementById('frame_compiled');
query_frame.document.write(compiledQuery);
}
"frame_compiled" is the name of my iframe:
<iframe name="frame_compiled" width="400" height="125">
</iframe>

I know that "compiledQuery" has been assigned a value (I used an alert with the value). Hence, I also know that the "prinRequest"-function works on principle... However, it seems as if I am having problem with retrieving the name of the iframe to the function (I tried doing an alert with the value, it was returned as null).

So, I suppose my question is this: How do I properly retrieve the value of the iframe, and am I on the right way using document.write to show the value of "compiledQuery" to the users? Or should I use another way of doing this completely?