If you hit CTRL-M you can get to your external device. From there you can load video..it has to be in a support non-DRM format: .mp4, .m4v, .m4a, .mp3, .ogv, .ogm, .ogg, .oga, .webm, .wav.
Taken from: Supported filetypes and filesystems - Chrome OS Help
Taken from: Supported filetypes and filesystems - Chrome OS Help