Set permissions for the USB-serial device using the command

sudo chmod 777 /dev/ttyUSB0

Note: Vic Pucknell reports that he has sometime observed the device
rotate between /dev/ttyUSB0, /dev/ttyUSB1, /dev/ttyUSB2 etc - make\
sure you are using the correct device name with the command:

ls -l /dev/ttyUSB*


Start PuTTY

Select Connection Type 'Serial' 
Enter Serial Line (/dev/ttyUSB0 or /dev/ttyUSB1)
Enter speed (9600)
Select 'Open'

Note: permissions for the USB-serial device will revert to their
default (mode 440) when the OS notices the change but by then
the serial line connection will have been established.