How do I Reference fields in a table when the table has been placed on a form?

Hello experts, here is my dilemma:  I am creating a form to receive payments.  As checks arrive I need to assign them to customer’s orders.  
The form I have created contains 2 sub forms.  The sub form on the right side of the screen contains customer orders that I have not been paid for yet (displayed in datasheet view).  
The sub form on the left side of the screen contains checks that have not been assigned to orders.  This “unassigned checks” sub form is actually a table (Is there such a thing as a sub table?).  I have to use a table on this side because often times I will receive a summary check that will pay for many different customers orders.  A sub form wasn’t allowing me to view the sub-data; in other words with a table I can see that check 1002 for $5000.00 includes payments for 50 customers which is stored in a “check details” table, a sub form would display check 1002 for $5000.00 without the 50 customer’s breakdown that I need.  
Here is where I am going with this…in the middle of my screen I have 2 text boxes (txtCustomerID & txtPaymentID) & a command button.  
When I click on the CustomerID field in my sub form (on the right side of my screen) the CustomerID value is displayed in txtCustomerId. I am looking for the same result in my “unassigned checks” sub-table.  
Goal 1 (worth 200 points): I would like to click on the PaymentID field within the “unassigned checks” sub-table & have that value sent to txtPaymentId.  I can’t seem to make it happen.
Goal 2 (worth 300 points): I would like to click the command button and have access find the paymentId value from txtPaymentID in my “unassigned checks” table and insert the value from txtCustomerID into the field that I have set up for it in the “unassigned checks” table.  Remember, that the paymentID may be in the sub data sheet table if it is part of a summary check as described above.
Here is the end result that I am looking for: Mouse click on an order, diplay the order number in a text box.  Mouse click on a payment, display the payment in a text box. Mouse click the command button under the text boxes to assign the payment with a customer.

Thanks!