You know about QuickSilver right.?

So you already know a brief explanation for QuickSilver. And to change your QuickSilver HotKey, go under QuickSilver Preference.

You can either click the preference symbol or using keyboard shortcut , to go to that window.

However, the preference symbol is such tiny that I think you will hardly notice it in a glimpse. It's here..

Now go under Command tab, you can find it hanged on the sidebar.

I think within a second you will see HotKey Activation bar. Yeah, you're right, it's the place for you to key in your most favorite hotkey. It's easy, isn't it?

Note that, most of QuickSilver users use Spc for the hotkey. Since it's already taken by Spotlight, I make my QuickSilver hotkey Space.