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.