If you signed up for the event through Xentry, you will be able to pay for the entry through Xentry.  
If you submitted your entry through a different event registration system, you won't  be able to use Xentry to pay for the event.

Entries are not considered complete until payment is received. If the close date has passed; it is at the discretion of the Event Secretary whether the entry will be accepted.  If you have any questions, feel free to call the USEA Office (703)779-0440.

If the event is still open, use the following steps to pay for your entry:

Log into USEA Online Services
Click the List, Print and Pay for Your Entries link.

On the Xentry List page, click the Details link on the entry you want to submit payment.

Click on  Submit Payment link.

Scroll to the bottom and enter your payment details & click on Pay your Entry Fee Electronically.

You will be emailed a receipt.