Paypal have recently changed their user interface in developers.paypal and as such, I couldn't find out the place from where I can log in to my old SANDBOX account.
In the new developer.paypal.com, on the Application tab, I selected the sandbox account,
In the following link,
https://developer.paypal.com/webapps/developer/applications/accounts
I get a button "login with paypal". But when I try to login, it displays a message "Invalid email" even though my email and password are totally correct.
The new developer accounts require that you first log in using a live PayPal account email address and password. Once you log in you can recover your test accounts by following the steps below.
Since you've had access to the legacy developer account you'll also need to delete your cookies and cache. Strangely enough, if you don't there will be several issues with logging into the account or accessing the test accounts. This will also resolve the invalid email error.
Let me know if you have any issues after this.