We will need to update the description there for the custom font because custom web fonts have to have all 4 files uploaded to display in browsers. You need all 4 files, see screenshot. You need .woff, .ttf, .eot, .svg
Also, please note that the custom font right now only works for Headings and menus. If you upload all 4 of the web font files, then the headings and menus will automatically display that font, you wont need to select it at all. Check out our video on the Museo Slab font, we show you the process of using a custom font.
If its a google font, then you can add it by following the steps in the thread below. We include all the google fonts but more are added all the time so we have to add more with an update, but you can add then before hand if you wish.
In a future update we will allow custom fonts to work with more than just headings and menus, and you will be able ot upload more than one custom font, but you will still need all 4 font files to do that, which is how web fonts work and browsers know how to display them.