The 'drop down tab' is called a sheet:
tell application "System Preferences"
activate
set the current pane to pane id "com.apple.preference.universalaccess"
reveal anchor "Mouse" of pane id "com.apple.preference.universalaccess"
tell application "System Events" to tell process "System Preferences"
click button 5 of window 1
click checkbox 1 of sheet 1 of window 1
end tell
end tell