You will not be able to have the output of any add-in graphics card route through the motherboard's VGA/DVI connector because there is no path to do so. The motherboard's VGA/DVI connector is only there to allow you to use the on-board integrated graphics.
Firstly, you should have your monitor hooked up to the Radeon card directly. Secondly, make sure that you have the card properly seated and that any external power connectors are plugged in so that the card is getting power.
If you still aren't seeing anything output from your Radeon card, hook your monitor back up to the integrated graphics and boot into the BIOS menu. Make sure that you select "external graphics adapter" (or whatever is similar) as your primary display device. Then hook the monitor back up to the graphics card and reboot.
You should be seeing output on the monitor from the Radeon card, now.
Note that when you are using a graphics card, you cannot use your integrated graphics alongside it. It's an either-or proposition.