Authorize.net Sandbox Account Marketpress

I am trying to utilize our Sandbox Authorize.net account to test credit card transaction within the marketpress plugin. I keep getting an error that states "There was a problem finalizing your purchase. (TESTMODE). Should the mode be switched to "Live" in order to use a full developer Sandbox account? I am trying to set it up so transaction are sent to our Sandbox Developer account so we can test the entire process.