Drop your video file into ~/Library/Application Support/iPhone Simulator/6.1/Media/DCIM/100APPLE/ and maybe delete the PhotoData file in Media.
Long Answer:
Make sure your movie file type is supported on the iPhone. I used a .mov
Open the Library. (YourUserName/Library) If you can't find
the file, it is hidden. Make it visible or user Terminal.
Navigate to ~/Library/Application Support/iPhone Simulator/
If you have multiple iOS Simulators pick the one your are developing
for currently. (I had 5.0, 5.1, 6.0 and 6.1)
Navigate to ~/Library/Application Support/iPhone Simulator/6.1/Media/DCIM/100APPLE/ and drop in your video file. If the 100APPLE directory doesn't exist, create it.
Sometimes you have
to delete the PhotoData file in Navigate to ~/Library/Application
Support/iPhone Simulator/6.1/Media/ to force the Simulator to
recognize the new file since you didn't add it through normal means.
Xcode 4.6.3 and MacOS 10.8.4 fyi. Haven't tried on Xcode 5.