Promoting the release of Command & Conquer 3 EA released Command & Conquer as freeware in 2007. It is therefore legal to download. The game used to be available from commandandconquer.com.
This is a DMG image of the Nod disc of Command & Conquer for Mac. To be burned to a CD or mounted in a virtual disc program.
The process of manually installing the game on a modern system is fairly involved. If you are not comfortable with using technical tools and want to be up and running easily check out the Command & Conquer installer instead.