I’ve googled some keywords but most talked about the “open file” dialog by the script. My target is the Osirix software, which I have to click the menu item “Open Database Folder” then choose a specific folder according to date.
So this is not the traditional “open dialog and wait for user selection”. I need to do something inside the dialog, including getting folder names and choosing the right folder. Anyone can give me some hints about this manipulation? 
Hi! Don’t have Osirix. This said couldn’t you use System Events to type text or path a bit like this:
try
tell application "System Events"
tell application process "Osirix"
set frontmost to true
click menu item "[Whatever menu item]" of menu "File" of menu bar item "File" of menu bar 1 -- or sth like that, make your own tests
delay 1 -- if any delay is needed
keystroke "d" using {shift down, command down} -- this would take you to the Desktop
delay 1
set {x, y} to position of button "[Whatever button name]" of window "[Whatever title]"
click at {x + 10, y + 10}
delay 1
keystroke folder_name -- a variable you'd defined earlier in the script
keystroke return -- if ever needed, just adapt to match your needs
delay 2
end tell
end tell
on error the error_message number the error_number
set the error_text to "Error: " & the error_number & ". " & the error_message
display dialog the error_text buttons {"OK"} default button 1
return the error_text
end try
Sorry to still code like an old boy.