Humm normally how i do it is as follows.
Download a TTF or make one.
Double click it so as to install it to the OS.
Open up the file Explorer and type shell:fonts (in the file path bar) make sure it is in the folder that pops up.
open up the content pipeline tool click add new item.
(there are two options to choose from at this point localized spritefont and spritefont).
click spritefont as above not the localized one name and create it.
open it up then type the name of the ttf in for the fontname
if that doesn't work it could be that the internal name of the font is different ive only seen this a few times.
In that case you can download a program called truetype explorer to see all the data in the font which is free but a obscure utility program. though you should be able to then see the actual font name inside you could then rename the file itself to its correct name.