You can only stream from your mac using iTunes. You can't 'push' video from a website for example.
For the home-sharing. Is your computer on and running iTunes? Are the videos in the correct format?
It needs to meet the compatibility of the ATV
- H.264 video up to 720p, 30 frames per second, Main Profile level 3.1 with AAC-LC audio up to 160 Kbps per channel, 48kHz, stereo audio in .m4v, .mp4, and .mov file formats
- MPEG-4 video up to 2.5 Mbps, 640 by 480 pixels, 30 frames per second, Simple Profile with AAC-LC audio up to 160 Kbps, 48kHz, stereo audio in .m4v, .mp4, and .mov file formats
- Motion JPEG (M-JPEG) up to 35 Mbps, 1280 by 720 pixels, 30 frames per second, audio in ulaw, PCM stereo audio in .avi file format
In iTunes you need to go to advanced - choose photos to share, if you haven't done so already.