Automatic installation (recommended)
OpenRA will prompt you to install the original game content the first time that you launch a mod. This process occurs automatically from inside the game: the game client can download a minimal set of game files from the internet, or copy the full game content (including music and movies) from an original install cd.
On the first run the game will ask whether you want to download a stripped down version automatically as both the full motion video and music soundtrack are optional. For the full experience you should install the complete asset set from the original game CD. Red Alert and Tiberian Dawn have been released by EA Games as freeware. You can download the CD-Images:
Please keep in mind that the CD extracting algorithm is hard-coded to those CD-Images. Localized installation CDs and expansion packs will not work. The BitLocker drive encryption feature has been reported to cause issues with the CD detection, so try removing any encrypted external drives if you encounter problems.
Manual installation (not recommended)
An unofficial guide to installing the files by hand can be found here.