You have to run this from the command prompt.  The program is called cmd.exe.
In Windows 7, you can get to it by clicking start and typing in cmd in the search box or navigate to Start > All Programs > Accessories > Command Prompt.

Card Type: HD6950 (unlocked)
Manufacturer: Gigabyte
Windows OS: Windows 7 Pro 64 bit
Setup: single card & single monitor running ClockTweak v0.7.4b

Works great, thanks  


C:\clocktweak>clocktweak -a
===============================================================================
                ClockTweak v0.7.4b - bitcoinX.com/clocktweak/
===============================================================================

Compatible GPUs found on these adapters:
Adapter#:0  ID:112690880  Name:"AMD Radeon HD 6900 Series"  Active:1  GPU:0

C:\clocktweak>clocktweak -r
===============================================================================
                ClockTweak v0.7.4b - bitcoinX.com/clocktweak/
===============================================================================

#0: Temp:53 Load:99 Fan:77 Level:2 CoreL2:800 MemL2:1250 mVoltL2:1100

C:\clocktweak>clocktweak -a 0 -m 300 -y
===============================================================================
                ClockTweak v0.7.4b - bitcoinX.com/clocktweak/
===============================================================================

Set values - starting.
 Adapter #0:
  setting mem speed: 300 [MHz]
Set values - finished.

Reading:
#0: Temp:56 Load: 0 Fan:77 Level:2 CoreL2:800 MemL2:300 mVoltL2:1100

C:\clocktweak>