1.) Download the video you want to convert. It must be 10 minutes or less for it to work on the 3DS.
Use this website to download your vids from youtube --> Save YouTube > Download YouTube Videos Online for Free
2.) Download and install Format Factory --> Format Factory - Free media file format converter
When you install, un-check the first check-box to refuse to install a pesky toolbar...
3.) Run Format Factory.
4.) Click the button that says "All to AVI".
5.) Click the "Output Setting" Button.
6.) Use these Settings --> http://i39.tinypic.com/243rxht.png
The video bitrate: 16000 can be lowered, but the lowered but you will loose video quality but the file size will be smaller as well.
7.) Click the "Save As" Button, and save it as something like "3DS 2D Video"
8.) Now click the "Add File" Button, and then add the video file you want to convert.
9.) After the file is on the list click the "OK" Button.
10.) Now Click the "Start" button and then your video is converted to be playable on the 3DS.
11.) All that's left is to rename the filename, to do that, just rename it to something like "HNI_0001" the number to the right of the HNI_ must be 4 digits and they must not be any of the same numbers as that's in your image/photo folder where your images are stored on your 3DS system.
12.) Copy the converted video onto the SD Card that's in your 3DS system in the folder where your videos/images are stored.
13.) now go to your Photo app on your 3DS and watch/enjoy it.
sos:http://3dsforums.com/nintendo-3ds-3/playing-downloaded-videos-your-3ds-2d-3d-29318/