Example code is included in the distribution file.
Here you can find the API documentation.
Note: When you get a -5 error "can't claim usb device" during ftdi_usb_open(), make sure the kernel ftdi_sio driver is unloaded.
You also find a patch for uisp here, an Atmel microcontroller flashing software, to program these via bitbang mode. The patch is not released yet and is only here as a bitbang mode example. Take a look at line 315+ of the patch for code using the bitbang mode.