I think you've just got you're slashes the wrong way around.
/ is the root of the web app
\ separates folders in physical windows paths
so try Folderpath=server.mappath("/") & "\batbin"
also in this line:
Set a = fs.CreateTextFile(Folderpath & Request.Form("Zip") & ".fdf")
make sure you have a \ between the batbin folder name and the file name.
Permissions can be set from Explorer. Just right-click the folder, choose Properties, go to the Security tab and add write permissions for whatever user your asp page is running under.