Ok - displaying my full noob colours here!

I'm trying to create the Protx payment module, based on the uc_authorizenet.module, and I am fairly sure I know what I need to do.
I have my new Protx module installed, and in use, but at the 'Review Order' page, submitting the order just returns me to the same page.
If I manually force a success in the module, it processes the order.
So I need to debug my function uc_protxvspdirect_charge($order_id, $amount, $data).
I have installed the devel.module but I can't seem to get the function to display any of it's variables on the page. I need to view what the response is from the POST request.
So how can I display variables and arrays on the page.
I have tried using dsm() dvr() dpr() but to no avail.
Am I missing something obvious or do I just not understand how to use the devel module?

Any help or pointers greatly appreciated.

Cheers!