I am having a problem setting up paypal chained payments...

Hi there! I am running a multisite marketpress site and have now gotten to the point of setting up paypal. I see from the number of posts on here that there are all sorts of problems that can occur. When I attempt to run a checkout on my site, I get this error:

There was a problem connecting to PayPal to setup your purchase. Please try again.
560022 - The X-PAYPAL-APPLICATION-ID header contains an invalid value

I have my API credentials and everything. I guess I just haven't configured something? I have read through the helpful tutorial that is referenced a lot on here; however, when I get to the link of "register and submit your application while logged in as developer" I click the link and it tells me server not found.

Argh...I'm bearing down and preparing for this to be an arduous process...