Extract the zip file to the USB stick attached to the Fritz!Box 7270, e.g. to \\Fritz!nas\fritz.nas\USB-FlashDisk-01
According to the documentation the CUL will be flashed automatically when FHEM is started via the script "startfhem". But the required dfu-programmer is not available on the Fritz!Box. In the log file you would find errors like this:
usb create starting
CULflash (dfu-programmer atmega32u4 erase && dfu-programmer atmega32u4 flash
./FHEM/CUL_V3.hex && dfu-programmer atmega32u4 start) 2>&1
CULflash sh: dfu-programmer: not found
usb create end
Therefore you have to flash the CUL separately on your PC:
Open the "Gerätemanager" (hardware device manager) on Windows and select the CUL. Note that the device is marked yellow, i.e. the driver was not correctly installed. Reinstall the driver by selecting the installation directory of FLIP as source directory for the driver. (If you don't reinstall the driver you will get an error message about a missing atlibusbdfu.dll later.)
Click "File - Load HEX file" and choose CUL_V3.hex which is provided in the zip file mentioned above, e.g. \\Fritz!nas\fritz.nas\USB-FlashDisk-01\fhem\FHEM\CUL_V3.hex
Click the buttons one after the other, e.g. choose the correct device (e.g. ATmega32U4 - at least for my Fritz!Box 7270), communication medium "USB", erase, program etc.
Close the FLIP program, remove the CUL from the USB and attach it to the Fritz!Box.
Now you can restart fhem via the script "startfhem".