I've done this in the past by emailing the UC registration folks ucregis@esri.comOr and then usually following up once they respond by phone (with my cc info). They have always been very responsive.
make sure to include you registration number in you email so they can locate you faster.
You can sign up for Preconference Seminars by calling our Registration team at 909-793-2853 ext. 1363. They'll be able to add any seminars to your registration.
