If the cash account that you want to set as your default is not listed in the Setup screen, review your Chart of Accounts to make sure that the account is set up as a Bank/Cash type account. You can add the account if it does not exist in your chart of accounts.

To do so, select Setup and click on Chart of Accounts Setup. Click on the Add New GL Account button and then select Bank/Cash from the list.

Continue through the steps to finish adding the account. The name of any account selected can always be edited in the last step to help with identifying it. The account should now be listed as an available option when you select the "Set Default Cash Account" link at the top of the page.