Head to a track’s editor page, or click on an individual track in an album editor - you’ll then see this “add video” link towards the bottom right of the page: 
What video file types do you accept?
Generally, we support any kind of video file (for example .mp4, .avi, .wmv, and .mov) and the vast majority of video and audio codecs (for example H.264, AAC, VP8, ProRes, WMV, WMA, Ogg and so on). However, we recommend you upload in MP4 and H264 − simply because the processing will be faster and you’ll obtain the best quality playback.
What is the maximum video upload size?
The max upload size for videos is 500MB, which works out to about 7 minutes at HD-quality. You’ll minimize your upload time and still get full HD by uploading MP4/H264 with a bitrate of roughly 5000 kb/s at 1920x1080 pixels (if you upload in a higher quality, your video will be converted and scaled down while we prepare it for playback on the web). If you do not have an editor available to re-output your video at this quality, a number of free tools are available, such as MPEG Streamclip, Handbrake or Miro Web Converter. Example of Handbrake settings 
How do I feature a video on the top of an album page?
To feature a video you’ve uploaded to a track at the top of the album page, click on the track in question on the left side of the album editor. Once the track is selected, scroll down to “video” on the right-hand side and check off “feature this video at the top of the album page.”
How do I upload a custom thumbnail?
Currently, we'll need to do that on our end. Please let us know if you would like one uploaded by contacting us. We'll respond back to request a copy of your thumbnail image. Make sure the image is sized at 1920 x 1080 pixels.