The IMEI (International Mobile Equipment Identity) is a 15 digit number unique to every mobile phone and mobile broadband modem. It's used to identify devices and can be used to prevent them from working if reported stolen.
The majority of modems have this number printed on the back. Some modems have the IMEI number hidden in the SIM card compartment or under the back cover, see images below.
The images below show common locations where a modem's Make, Model and IMEI number can be found
IMEI Printed on Back Cover
IMEI Printed in SIM compartment
IMEI Printed Under Back Cover
IMEI Printed in Battery Compartment
If you are unable to find your dongle IMEI number, please watch this YouTube video, showing how to find the IMEI number using Windows.