Have Automator folder action pass a list of files to AppleScript
Use a run handler:

on run {input, parameters}
    input
end run


Or if you need the input as a list of POSIX paths:

on run {input, parameters}
    set l to {}
    repeat with f in input
        set end of l to POSIX path of f
    end repeat
end run


For a script saved directly in /Library/Scripts/Folder Action Scripts/, use an adding folder items handler:

on adding folder items to this_folder after receiving added_items
    added_items
end addin