Just went to FIOS today. When setting up Windows Media Center to use my CHS 333HDC, WMC changes the channels successfully. But when selecting the program guide, FIOS is not an option. Only five guides are available including COMCAST and a few others I never heard of. USA is the region and my zip code is correct for my home in DC.
i guess I found a workaround. I used the ZIP code for a location in VA that I knew had FIOS. That presented an option for FIOS. I'm assuming that my current location is unknown for for FIOS by either Verizon or Microsoft.