Yes: it's always a good idea to provide some info about the context. ;-)
So, 10.4.11.
The print sheet displayed by Safari (and provided by the system) has another layout, so that the pop up button of interest is the second one.
On the other hand, it seems that it is mandatory for the target application to be active.
As a result, this one seems to do the job here, with Safari 4.0.3:
tell application "Safari" to activate
tell application "System Events"
tell application process "Safari"
tell front window
tell front sheet
tell pop up button 2
-- Display the local menu
perform action "AXPress"
tell menu 1
-- Q2: get available "values"
name of menu items
-- Q1: select an option (here, "Save as
")
tell menu item "Enregistrer sous
"
perform action "AXPress"
end tell
end tell
end tell
end tell
end tell
end tell
end tell
Of course, as you did in your sample snippet, I'm assuming the printing sheet is already displayed.
HTH,
Axel