How do i drain a transmission (buick 400...

You have to remove the tranny pan in order to drain it and to change your filter. Why no manufacturer has ever incorporated a drain plug for the tranny.....I'll never know! Here's a diagram to help you out.
(Copy & paste entire link into your address bar)

http://www.autozone.com/autozone/catalog/parts/partsShelf.jsp?displayName=Transmission+Pan+Bolt+-+Performance&currentPage=1&categoryDisplayName=Drivetrain&navValue=11200825&itemId=825-0&parentId=12-0

Take the pan off.