I Pasted API user name,password and signature and saved the settings. I set mode as Live. Now when I visit a sub-site and click Account Upgrade, It shows 404 error page.
I followed the below steps given in paypal website:-
1.Check your account status. Login to PayPal. Go to your PayPal Profile and click My settings. Confirm that your Account Type is either Premier or Business, or upgrade your account.
2.Check your API settings. Click My selling tools. Expand Selling online if needed and check API access. Click Update and Add or edit API permission or View API signature.
I pasted API username,passsword and Signature in Pro sites settings tab.