EDIT: if you are not the license/account administrator, see Karen's comment below. -rs
You need to find the person that manages your licenses and has the administration access to myesri.com
There is an Events tab on that page
Or if you are the administrator, you may want to talk to your customer service rep or local distributor. Complimentary registration for the User Conference are typically tied to annual maintenance, although there may be others ways that I am not aware of (again, talk to you customer rep)
By the way, this is for the User Conference only. Developer Summit do not have any complimentary seats, typically, and are not listed on this page. Other conferences typically are not either, at least that I have ever seen.