jrako - first, if you have multiple pay options created, make sure you are setting up the auth.net account info for the pay option that is assigned to your event that you are testing with.
Second, assuming you have the data in the right place, if you still have that error, it's not a DT Register problem... it's incorrect account info just like the error says. You'll need to contact Authorize.net and proceed with them. They can confirm if you are using the correct data. If you don't get anywhere, submit a ticket and we'll check to make sure you are setting everything up correctly on the DT Reg side. But, this error would only happen if the account info is missing, incorrect, or there's an issue with your account itself.
I was having the "Invalid API or inactive account" issue with my site for a while. I searched all over trying to find out what was wrong. Changed my authorize.net API key 6 times and called my CC provider to see if something was amiss. Then I turned "Test" mode off in the DT backend. Problem solved.