Concert tickets are usually very tricky because they are, in a sense, bearer bond like instruments. In other words, as long as a person has a ticket, it doesn't matter who they are, they get to use the ticket.
Thus, in order to cancel a ticket, you need to be able to specifically identify the tickets. You need to look through the information you have to determine whether there is any ticket number which identifies the tickets, so that you can ask for the lost tickets to be cancelled and re-issued to you.
Your next issue is that you are dealing with a ticketing agent. You need to see if you can contact the ticket issuer, which in this case seems to be Ticketmaster. the process for reissuing tickets is at: http://www.ticketmaster.com/h/lost_tix.html?tm_link=help_nav_1_lost
Ticket agents will purchase tickets from ticketmaster and then will hold them out for sale to the public. Really, the ticket agent should do this for you. However, the only way you can force them to do this is to sue them. Since you are on a tight schedule, that probably wouldn't resolve your issue.
The ticket agent is not being honest with you. I would write a stern letter threatening a lawsuit for misrepresentation under the California deceptive trade practice
laws under California Business and Professions Code § 17500 et seq. You then need to ask for the specific identification information on the tickets, so that you can contact ticketmaster yourself and get the tickets reissued. But first, demand that the ticket agent do this for you.
If all else fails, you can sue in small claims
court to recover the loss of the tickets, claiming that the agent failed to do as you requested and help you get reissued tickets.
Please let me know if you need clarification.
Zachary D. Norris