Some venues using Horse Monkey for online entries also have a show administration version as well so they can keep track of rider account transactions and manage them internally, these accounts are held directly with them (the show organisers) and not linked to individual Horse Monkey riders accounts.

The credit is held within the venues own Horse Monkey system rather than a credit on the global Horse Monkey website which will be why you are unable to see the credit within your account and why a refund has not been processed through your Horse Monkey account.  

On this occasion it would be best to contact the Show Organiser directly so they can advise you on how they can process this for you.