How to put PayPal Test Button on a site?

I'm trying to follow these instructions. I log in to developer.paypal.com, then to http://ift.tt/U9tsBr with a Business account from developer.paypal.com's Sandbox Test Accounts page. Go to Merchant Services, click "My saved buttons". Domain changes to www.paypal.com (is it as it should be?). I choose Buy Now button, specify item name and price, and click Create Button. Remove code protection, copy the code, paste it to a site. Open the page, click Buy Now button, which gets me to www.paypal.com, specify Personal account's credentials from developer.paypal.com's Sandbox Test Accounts page. Click Log In, but it says:

Please check your email address and password and try again

Please make sure you have entered your email address correctly.

Please re-enter your PayPal password.

And yep, I checked if I can log in with those credentials to http://ift.tt/U9tsBr. What am I doing wrong?