PayPal Not Working - First time setup

I've installed DMS, and entered my Paypal ID (username), but when testing out a transaction, I just keep getting an Error 500 from Paypal.
url: www.sandbox.paypal.com/cgi-bin/webscr

Is there other configuration that needs to be done with the Paypal account?
Am I missing a step?

Please advise.

Hi

You don't need to do any setup with your Paypal account . Could you please send me your site URL so that I can check ? I noticed that you are using test mode . Maybe Paypal dev server was not available at the time you are testing ? I didn't test the extension with test mode but I am sure that It works fine in live mode .

Thanks ,

Tuan