It's on by default on OS X Lion+.

You need OS X 10.7 or later. To enable:

    defaults write -g ApplePressAndHoldEnabled -bool true

To disable:

    defaults write -g ApplePressAndHoldEnabled -bool false

I was going to say that on OS X 10.6 and earlier, you can type Option+E, then type C, but it turns out that doesn’t work. Option+E only lets you add the ´ to vowels, like Á. I confirmed that using the Keyboard Viewer (search for it in System Preferences).

However, you can create a custom keyboard layout with Ukelele (http://scripts.sil.org/ukelele), in which you replace some character you never use (option+something) with Ć. I use a custom layout myself, in which I swapped some keys around so it’s easier to type smart quotes ‘ ’ “ ”.