Multii Processor Mode can not be activated

I use Nuendo 5.5.3 Build 143 with a Win7 Rechner and Intel Core2 Duo CPUs E8400, that are both working due to the task manager. But I can not activate the Multi Processor Mode in Nuendo (Checkbox is grey). How can I solve this problem? ASIO-Hardware is an Alesis I|O 14 via Firewire.

Check out video player settings. Boost video means using one core for video only so you can’t enable MP with 2 cores.

Thank You, Rustami, for the great hint. Now things run much better.