where is arrangement in display

the arrangement tab page only appear on the primary display, which would be the mac if you attempt to connect the external monitor because there is something wrong with the imac display, then you need to click on the gather windows to collect each of the display dialogs which appear on their repsective display to the display you wish to use to alter them.

If you have downloaded third-party USB drivers, the battle is lost already, as no third-party drivers should be necessary to connect to an external display via USB <> VGA Adapter cable, and if so, those drivers may be in conflict with Apple's USB detection process for your external display.