Your Aiwa receiver is digital-ready, meaning it can accept 5.1 channel analog inputs but it cannot decode the 5.1 digital format on its own.
As others have said, to get 5.1 using this receiver you would need a DVD player with 5.1 channel analog outputs. Either tha tor you need to buy a separate Dolby Digital decoder that accepts digital inputs and has 5.1 channel analog outputs (assuming such a thing exists).
I'm in the same situation right now and that is why I am looking to upgrade my receiver.
Edit: if the Aiwa does have a coax input, maybe that converter would work.