The Credit Memo record (and possibly other records) have an Account field, where you can choose which Accounts Receivable account will be affected by the Credit Memo. However, this field has somewhat unusual behaviors.
First of all, the field won't show up on Credit Memos if you only have one Accounts Receivable account in your Chart of Account. This is true even if you're using a custom Credit Memo form and the "Show" checkbox for the "Account" field is checked. As soon as you have two or more accounts of type "Accounts Receivable", the drop-down will show up. This can be quite confusing, especially since NetSuite usually shows drop-downs with only one result and allows you to create new records (such as an account in this case) on-the-fly with the little "+" button beside the drop-down.
Second, the default account used for Credit Memos is not a company setting, unlike most accounts for transactions. In the case of Credit Memos, the account is what we call a "sticky" default, which means the default account that is selected on the Credit Memo is the one used on the last created Credit Memo. This means you must always check the account on the Credit Memos you create to make sure it's the correct one.