Problems with UI control of Preview

This is my first time playing with UI control, and I want to open a file in Preview and fax it. Just trying to click Open seems to be a problem. Here is my script:

tell application “Preview”
end tell

tell application “System Events”
if UI elements enabled then
tell process “Preview”
tell menu bar 1
click menu item “Open” of menu “File”
end tell
end tell
tell application “System Preferences”
set current pane to pane “”
display dialog “UI element scripting is not enabled. Check “Enable access for assistive devices””
end tell
end if
end tell

and the result when I try to run it is this:

“AppleScript Error
System Events got an error:
NSRecieverEvaluatorScriptError: 4”

any ideas on why this is?

It should be:

menu item "Open…"  of menu "File"  of menu bar item "File"  of menu bar 1

(The three-dots in “Open…” are a single character, opt+dot)
But much more quicker if you don’t use so to open a file:

tell app "Finder" to open alias "path:to:file.pdf" using application file id "prvw"

Small nitpick: it is actually option+; (semicolon).