Here are a couple of ways (there are probably more)
In your drawing there are two small squares one behind another, one set to pen by layer, the other set to pen white, as you mentioned layers - I'll assume you mean the one set to pen by layer.
First thing to note is you need the Brush transparency set to 0% not 100%, also if you wish a solid white fill, on a white background screen, you need to turn off 'Convert color in accordance with a current background color' (drop down list in both pen and brush properties - select 'no')
You can either select the square and use format bring to front, or you can use layer order, put the square on a new layer and set its layer order number to a higher number than the rest.http://www3.turbocadcommunity.com/tiki-index.php?page=Layers
about a third way down the page