I have just spent 5 hours trying to create test buttons in the PayPal sandbox but it keeps diverting me to the live site. The web is full of people with the same issue and it was reported in June 2012.

However there is a workaround.

PayPal’s response,

Re: Paypal sandbox – merchant tab takes you to live paypal site unable to create test buttons

I know this has been an ongoing issue with the Sandbox.  We apologize for any inconvenience this has caused. I don’t have an ETA for when it will be fixed but thank you all for reporting this on the community as it helps us get a sense of how many people are affected.

You can access the PayPal button creator/manager from the Profile settings in the sandbox account.  I’ve also included the direct link below:

Create a new button: https://www.sandbox.paypal.com/us/cgi-bin/webscr?cmd=_button-designer

Manage existing saved buttons: https://www.sandbox.paypal.com/us/cgi-bin/webscr?cmd=_button-management