MACROMEDIA FLEX-FLEX ACTIONSCRIPT LANGUAGE Reference page 176

// AS file is in a directory at the same level as the directory
// that contains the script file
// The directory is named "ALL_includes"
#include "../ALL_includes/init_script.as"
// AS file is specified by an absolute path in Windows
// Note use of forward slashes, not backslashes
#include "C:/Flash_scripts/init_script.as"
// AS file is specified by an absolute path on Macintosh
#include "Mac HD:Flash_scripts:init_script.as"
See also
import
176 Chapter 5: ActionScript Core Language Elements