Download the drivers from:
http://support.amd.com/us/gpudownload/Pages/index.aspx
Latest version is 9.11.
Make sure you're running BOINC 6.10.x and that you didn't select the "protected application execution" option during installation. When you start BOINC it should say something along the lines of "ATI GPU 0: ATI Radeon HD 4700/4800 (RV740/RV770) (CAL version 1.4.467, 512MB, 1200 GFLOPS peak)" in the messages.
I'm not sure if Milkyway is distributing the ATI GPU app automatically yet, so you might have to set that up manually.
edit: also, none of this will work over a Remote Desktop Connection or VNC.