Need help getting plugins to work :(

i am hosted over at godaddy.com. I originally have wordpress.com blog, then updated it with the download from www.ads-software.com. I did this because I want to add plugins (like a shopping cart) to the website. Now I am completely lost. It says it’s all updated but my dashboard looks the same. I have no Plugins on the sidebar and no way of figuring out if they are there or available. I am pretty much a novice at this and the computer speak has me going cross-eyed. Any help would be very welcomed and much appreciated.