I need to get a report of the total amount of payments that was collected during our last event. Where can I export this report?

You can download your form submissions to excel so you can generate a report.

https://www.jotform.com/help/73-How-to-Download-Form-Submissions-as-Excel-CSV-PDF

If you are using payment gateways like Paypal, you can also generate reports on your Paypal's dashboard.

You can also use this payment app to get an overview of your collected payments.

http://apps.jotform.com/app/jotpayments

Hope this helps!