Mixcraft uses Microsoft DirectShow to decode videos. Make sure Windows and your graphics drivers are up to date. By default, Mixcraft recognizes WMV and most AVI files. If you can't get a video file in another format to load into Mixcraft, you could convert it to WMV. You could use a video conversion program to convert the video to WMV.

Alternatively, you could install a video codec pack like K-Lite and that would allow Mixcraft to directly load the video files. However, some codec packs are full of malware, so be careful what you download and peruse each step of the installation process carefully for unwanted bonus software additions to your computer.