You have to change Settings in MSI Afterburner:

Go to MSIAfterburner.cfg located in MSI Afterburner Directory.
Open It.

replace

[ATIADLHAL]
UnofficialOverclockingEULA   =
UnofficialOverclockingMode   = 0

with

[ATIADLHAL]
UnofficialOverclockingEULA   = I confirm that I am aware of unofficial overclocking limitations and fully understand that MSI will not provide me any support on it
UnofficialOverclockingMode   = 1

also search  for

UnlockVoltageControl           = 0
UnlockVoltageMonitoring      = 0

and replace with
UnlockVoltageControl           = 1
UnlockVoltageMonitoring      = 1

And now have fun with Overclocking

Greetings,
Frog