The following links will help you. You need to fill in DS-156 Eform on clicking continue it will generate the barcode. Then you need to take print out of the bar coded PDF and send it to St. Louis.
Hope this helps
http://www.murthy.com/UDnds156.html
http://travel.state.gov/hlop.html
http://evisaforms.state.gov/
https://evisaforms.state.gov/ds156.asp?lang=1