Have you installed driver of the GPU on Windows 8? Also check in your mobo BIOS setting if it is primary display adapter. If not then set it. If setting is automatic then connect the display cable on the external card and it should give the display.
In addition to EasyLover if you are not very technical and you are unsure how to install and update the driver just go to AMD website in the downloads section there is a download that will auto detect your adapter and install the correct driver for your new OS.