I've seen other postings where people can't load "My Selling Tools" and I happen to be in the same boat.
Support hasn't responded yet and I'm hoping to do a demo Monday of the Windows 8/Paypal API integration from http://paypal.github.io/Windows8SDK/ into WinRT apps - hence turning to may favorite net community, stack overflow :)
The link above gives a sandbox account to allow for third party access, so trying to add that account to allow the third party access but can't even bring up the selling tools to do so. I was able to bring up the selling tools from my main login, but not from within the sandbox login.
Once I login to the sandbox environment and try to access "My Selling Tools" it just hangs. The browser doesn't matter, same result across browsers. I get nothing returned but the wait image. Actually anything on the left hand side hangs not just the selling tools. I've tried more than ten times all throughout the day.
Of course, the hope here is that someone from the PayPal Technical team replies.
I can't wait another hour on hold, I just can't.