reboot your system first. sometimes 2.1 detection is wonky
then run something called gpu caps viewer, google it if you dont 100% know you have it
go to opencl tab. the very top dropbox should have 2 options if you have 2 sdks installed. 2.1 is called ATI stream. 2.6 will have a version number of something like 831.4. 2.5 will have like 793 or something, starts with a 7.
Thanks and CGMiner will auto detect and use SDK 2.1?
No idea, never used it. If its anything like diablominer though, it'll iterate through all opencl platforms and devices, so if you have 4 cards and 2 sdk's, it'll run 8 miners: half on 2.1 sdk and the other half on whatever other sdk you have installed; you'll likely need to specify what platform/devices to use.