Make sure your video input is connected to Input 2 of the IOXT. I remember that being quite the gotcha.
Since you are using XPression Prime you will only be able to overlay graphics on the video input in internal keying mode. XPression Prime does not support the 'Live Input" material that allows for manipulation of the video input. You'll need to upgrade to Studio if you want that.
If that doesn't work please include your XPression Prime version and build and the AJA driver version that is installed.
#XPression