I know of no sum option in any program. DSS lets you select medium or average as default. The whole idea of multiple images is to reduce the noise. A single exposure of the right lenght will show you the object but multiple images averages out the noise to make the image cleaner.
You will need to post process this image in photoshop to see the detail. I had a look at your jpg and the data is there, although it's badly distorted here due to size reduction, you just can't see it.
Here is a quick guide from an expert :-
http://www.ezystyles.com.au/photoshop.html