You may be able to press that button by saying,
tell application "System Events"
tell process "QuarkXPress"
keystroke (ASCII character 13)
end tell
end tell
ASCII character 13 is (I think) the return key. If a button is highlighted in a dialog (and Quark is generating the dialog), this may work. If you've got 4200 lines of Applescript you must know a lot more about it than I do, but this did work for me when other efforts failed (in QXP 6.5, OS X 10.4.9).