Your "iLok User ID" is the same as the "User ID" that you use to log into the iLok License Manager application or iLok.com.
When registering iLok protected software, your iLok User ID is required so that a license can be deposited in your iLok.com user account (and subsequently downloaded to your iLok USB key).
If you do not know for sure what your iLok.com User ID is, you should go to iLok.com and verify that you have the correct User ID. If you have not previously created a user account at iLok.com, you will need to do that first in order to get your User ID.
What is my User ID at iLok.com?
Your iLok.com User ID is sometimes referred to as your "iLok ID", "iLok User ID", or "iLok Account". All of these are the same thing, your User ID that you use to log into the iLok License Manager.
If you have not already done so, create a free account at iLok.com. Once you create an account at iLok.com, your User ID is what you use to login to your account on iLok.com or iLok License Manager (not your login password, which you also choose).
When purchasing or registering products on the Avid website or development partner websites that require an iLok.com account, you may be asked to enter your iLok ID, iLok User ID, or iLok account name. Again, all of these are asking you to enter the User ID that you create at iLok.com
Your iLok User ID is not the manufacturing part number or the serial number printed on your iLok. You would choose your own iLok.com User ID and whatever password you want. If you have already created an account on ilok.com, but have forgotten your password to that account, there is a "password reminder" link on the login page.