So, I downloaded DCS World and installed it.
Then I downloaded DCS P-51 and went to install it. Fail!
The problem was, I installed version 1.2.6.18933 of DCS World and the version I downloaded of DCS P-51 was 1.2.6.18651. DCS wouldn't let me install a older version module into a new version of DCS World. Naturally.
So, I went hunting for a solution and found it.
At some point this issue is going to pop up for someone else, so I thought I post a ED Forum link to the solution here to help everyone else out.
http://forums.eagle.ru/showthread.php?t=114030