There is no need to have FBReader on a Desktop computer in order to use it on the 770. Just transfer the e-books to the 770 via a USB connection (the 770 will show up like a USB memory stick). FBReader itself is typically installed via WiFi. FBReader will automatically find e-books in some metadata-rich formats (in it's search path), but for others you use FBReader's own file navigator to manually add e-books to its library. All this can be done on the 770. There is a 2-part visual tutorial actually from a 770 under
FBReader ScreenShots.
The only advantage of installing FBReader on a Desktop is that it allows you to read the e-books there, just as on your 770. Note that there is a
FBReader port for OSX 10.4 and above on Intel.