Did you read somewhere where FiOS was not going to carry Mundo Fox?
Mundo Fox is a network, similar to ABC, NBC, etc., that broadcasts via affiliates (i.e. local stations) as opposed to networks like TBS, Syfy, Comedy Central that broadcast only to satellite and cable providers.
I checked the Mundo Fox website http://www.mundofox.com/ (I instructed Google Chrome to translate it to English for me) and there's a list of cities in the top right of the page which will show the local stations that carry Mundo Fox.
If there's not a local station in your area carrying the Mundo Fox network then FiOS doesn't have a way to include it in their channel lineup for you.
Check and see if your city/area is included. My area, Dallas-Ft. Worth, has Mundo Fox as it is being broadcast on a local station (KFWD - ch. 14 in SD/ch. 514 in HD) which FiOS had previously included in their channel lineup.
