In Media Express go under edit > preferences. I had NTSC selected, not an HD signal (I switched it to HD 720p 60, the video then showed fine). Note: This setting has to match the same as what you’re capturing from. In my case, my 360 was set for 720p, therefore HD 720p was the correct setting.
Also, BlackMagic installs a control panel under your computers control panel. To find this (in XP or Vista) go to Control Panel and select “classic view” in the top left. It’s called BlackMagic Control Panel. In here you can change input settings such as HDMI or Component, etc.
Hope this helps.