a.) It’s easy to use
b.) It’s cheap (read: FREE)
However, for some reason, the preview screen only showed black. I dug out Google and Bing and began looking for solutions. I saw all kinds of solutions from uninstalling/reinstalling everything to using another app to import, convert, and export the Fraps movie to a format that WLMM could render. I know when I made my iRacing vids, I didn’t have this problem. So, I kept digging.
Finally, I happened upon a post on Windows 7 Forums that struck a chord. I have two Nvidia graphics card and after an upgrade to the 285.62 drivers, DCS: A-10 Warthog had some wierd graphical anomalies. There is a bug in this revision of the driver that causes chunks of the world to either render black or transparent.
So, with that in mind, I hit Nvidia’s website and downloaded the previous WHQL drivers (280.26) and voila! No more black preview screen. So, if you are having this problem I recommend getting the 280.62 drivers, or you can play around and find the latest drivers that fix this issue.