Gottcha. The MDAT system is still in Beta testing, so I suspect there may be something that isn't working quite right. (Colleagues and PRB and I also could not get results.)
So for now I would suggest downloading the PUMS tables and using STATA.
Please also submit a comment about this issue to
cedsci.feedback@census.gov Census Bureau staff take user input into consideration as they're working on upgrades and new releases of their tools, and they really do appreciate a heads-up when something doesn't work.