Nin.com does not host any images or video. Instead, it imports images and video from your Flickr and YouTube accounts. To add videos to nin.com, you'll first need to add those images to your YouTube account.

Make sure you have authenticated your YouTube account with nin.com. Do this by editing the "Contact Info" section of your profile, and following the instructions at the bottom of the page (view detailed instructions).
In the nin.com video galleries, use the pull-down menus at the top of the page to locate the gallery you wish to add a video to.
    
On the main gallery page, just underneath the gallery's title, you will see "YouTube code" followed by a string of letters and numbers. Each gallery has its own unique code, and any of your YouTube videos you tag with that code will show up in its corresponding gallery. Highlight the code and copy it using Ctrl+C (command+C on a Mac), or write it down.
    
Unfortunately it can take up to a day for YouTube videos to show up on nin.com, so please be patient. Consult the FAQ if your videos don't show up on nin.com after 24 hours.

Still confused? View the step-by-step guide to adding videos.