YMCAC (YMCA: Classic) is a mod of an OpenRA mod called YMCA. It updates an old version of the mod, adding new maps, balances and some other cool stuff.
As per usual with other OpenRA mods, download the installers in the release section.
If the content installer sequence fails, download the content from here: https://www.mediafire.com/file/j25l2j8dnoa0avs/ymcac.zip/file To add the content to the game, stick the folder inside of the archive in the \AppData\Roaming\OpenRA\Content folder. If done correctly the game should boot without needing to use the content installer.
(Finally got those damned installers to work if you couldn't already tell.)