It was very hard to join Paypal Developer Central. I registered for access long time ago. After registration, it said it would send me an email to activate my account. But I did not receive this email. I requested a resend but nothing happened either. I asked for Paypal Technical Support to look into this matter but they only suggested me look at my spam folder.
Today by chance I requested a resend of activation, and I got it. So, it was likely that Paypal had a Email System problem for a certain period.