First of all you have to check if the device is connected to the system
lsusb
The output will be something like
Bus 002 Device 006: ID 046d:c05a Logitech, Inc. M90/M100 Optical Mouse Bus 002 Device 005: ID 046d:081b Logitech, Inc. Webcam C310 Bus 002 Device 004: ID 046d:0a0b Logitech, Inc. ClearChat Pro USB Bus 002 Device 003: ID 413c:2513 Dell Computer Corp. internal USB Hub of E-Port Replicator Bus 002 Device 002: ID 8087:0024 Intel Corp. Integrated Rate Matching Hub Bus 002 Device 001: ID 1d6b:0002 Linux Foundation 2.0 root hub Bus 001 Device 004: ID 0c45:643f Microdia Bus 001 Device 003: ID 413c:8197 Dell Computer Corp. Bus 001 Device 002: ID 8087:0024 Intel Corp. Integrated Rate Matching Hub Bus 001 Device 001: ID 1d6b:0002 Linux Foundation 2.0 root hub Bus 004 Device 001: ID 1d6b:0003 Linux Foundation 3.0 root hub Bus 003 Device 007: ID 1608:0240 Inside Out Networks [hex] Edgeport/1 Bus 003 Device 006: ID 413c:2011 Dell Computer Corp. Multimedia Pro Keyboard Bus 003 Device 004: ID 413c:1005 Dell Computer Corp. Multimedia Pro Keyboard Hub Bus 003 Device 003: ID 413c:2513 Dell Computer Corp. internal USB Hub of E-Port Replicator Bus 003 Device 002: ID 046d:c52f Logitech, Inc. Unifying Receiver Bus 003 Device 001: ID 1d6b:0002 Linux Foundation 2.0 root hub
So, search for the device you know is the usb to serial. In my case..
Bus 003 Device 007: ID 1608:0240 Inside Out Networks [hex] Edgeport/1
Now load the kernel module using this command (check that the vendor and product are taken from the ID in the previous line)
sudo modprobe usbserial vendor=0x1608 product=0x0240
You are good to go! Use minicom or some software like that to connect to the serial device.