What is it you can't do. Once logged all you need is a simple "command" like this GET:

http://docs.google.com/MiscCommands?command=saveasdoc&exportformat=$FO
+RMAT&docID=$DOC_ID&hl=en
[download]

Valid export formats for $FORMAT include 'rtf' 'pdf' 'txt' and 'doc'. Obviously you need to specify the $DOC_ID you want.