I've downloaded X Pro and installed it. Cornerstone didn't show up as a plugin and it didn't appear as an extension. When I looked at the compressed file I saw Cornerstone in it so pulled it out, compressed it, and uploaded it as a separate plugin. When I attempted to activate said plugin though I received a warning that said "Cannot redeclare class Cornerstone_Plugin_Base in..."
Is there something that I am doing wrong or missing?