I just added DistRTgen to one of my Windows 8 Enterprise machines. No problems downloading the app or the kernels.
Was the DistRTgen folder added to you BOINC "projects" folder? i.e. C:\ProgramData\BOINC\projects\boinc.freerainbowtables.com_distrtgen
First thing I would check is: make sure Windows, A/V program, or firewall are not blocking the download.
Other than that not sure what the problem could be. I will try and add it to one of my Windows 8.1 Pro machines, and let you know if I have any issues.
*EDIT* I added DiRT to a Windows 8.1 machine. No problems downloading the DiRT app. Machine is running Windows 8.1 and BOINC client 7.0.64.