Your computer can not read the codec. The easiest way to fix this is to Install / Update Quicktime.
The MOV files are encoded with the PNG codec. Some systems are not natively configured to read the PNG codec and will show an error or replace the alpha channel with black. There are a few ways to get the codec loaded in your environment, you can either install or update Quicktime, or you can install the k-lite codec pack on windows or Perian on mac. The newest version of AE already has the PNG codec built in.
Getting the codec loaded is the preferred way to go, however you can alternatively use a video transcoder such as Handbrake or MPEG Streamclip to convert the files to another format supporting the alpha channel that works with your system.Install / Update Quicktime