http://www.squared5.com/svideo/mpeg-streamclip-mac.html

i just downloaded it and it seems to work.
Just open a MP4/AVI file on your computer then
go to file- export AVI
best downscale and in the customized resolution have
400x240p then select convert.
make sure the files are no longer than 9:59 in length
or else the 3DS wont read the file *its nintendos own software protection*

now you can have video on the go
perfect for segmented Anime off of youtube.

If you dont know a placce to download videos you can always use firefox
then on youtube click the download box/ mp4

pic for ref
http://i.imgur.com/4j8qL.png