As an admin, you might have been looking for a specific field at the account level and cannot find it on Reveal's custom fields list ?
Here are the main reasons why it happens and how to solve the issue!
Integration user
Your integration user might not have access to that field, hence why it does not appear on your data source custom fields list. Check your integration users rights and permissions and make the relevant changes.
Field is related to a specific object Reveal does not support
Custom fields will only show you fields at the account and at the opportunity level.
For example, RecordType is a proper object type in Salesforce. When you see the record type at the account level, it means what you are seeing is a "lookup field", a.k.a. a reference to one of the RecordType objects in the CRM.
To workaround this, you can create a computed field on the Account with the "Name" property of the related record type.