You will want to connect the outputs of the video switcher to the capture card on a computer. This will vary depending on your computer.
Ustream Producer is designed to work with the Blackmagic capture cards (but not only with Blackmagic cards).
We strongly recommend you check for hardware compatibility with capture card manufacturers before purchase.
In general, the following Blackmagic cards should work.
Blackmagic Intensity Pro - HDMI
Blackmagic Decklink SDI - SD/HD SDI
Blackmagic Decklink Duo - SD/HD SDI
Blackmagic Decklink Studio - SDI HDMI
Blackmagic Decklink HD Extreme 3D - SDI, HDMI
Blackmagic Intensity Shuttle - HDMI
If you have a Firewire port you could use the Grass Valley Canapous ADVC 110.