1 - Connect the the usb-serial cable,
2- check with dmesg which tty ID is getting assocated to it, for example /dev/TTYS0
(note: if you remove and reinser the cable a different tty ID can be associated)
3- change device ownsership issuing a chmod 777 /dev/ttyXYZ (or open fldigi with sudo from command line)
4- open in fldigi
keep the other parameters as per the presetting.