One nice thing about the eCart server behaviors for the Store Order Summary and Store Order Details is that they can be applied stand alone on your confirm page after the fact if need be.
I think that you should be able to select the tables from the connection within the checkout wizard though. As one more test of this please try the following.
On a new blank page create a recordset, in the recordset interface check on the connection and click the test button, it should tell you the connection is good. Then back in the recordset interface select a table from the connection and click on the test button here. It should display the records from the table. Once you have done this you can cancel out of the recordset interface and then try to immediately use the checkout wizard.
I have seen an issue in the past where a use was having a similar problem with another tool and just opening these interfaces and testing the connection and recordset allowed them to proceed in the wizard. Please post back and let us know if this solves the problem or if it persists and we can try something else.