This topic discusses using trust accounts to enter or receive a payment for an invoice. For a more in depth discussion on the payments screen, and on how to enter payment, please see our Payment section. The Enter Payments help topic is a probably a good place to start.
You will not be able to use any of the trust accounting features if you don't turn the option on in Settings->General Screen as shown below.
On the Enter Payment screen, once you select the client, you'll be able to see the total dollars available in the client's trust account. By selecting the Trust Account payment type, not only will the payment be received (which will update your aging report and the client's current balances), but it will also be added to the trust register and will update the client's current trust balance.