When you first registered on the Developer Center, you should have received a welcome email. Within that email, there is a link you can click on to request for sandbox credentials. You will then receive a package that includes the account details, the RSA keys and a sample configuration.
Of course, Yong I received, the sandbox credentials and a package that include the account details.
But, on the public Pod, I don't see where I will past the public key, I mean the adminitration link to access the setting part.
there is no need for you to do that. when that package was generated, the public key was already uploaded aginst the respective service account. also, this account is for the developer sandbox (develop2.symphony.com) and not the public pod (my.symphony.com) which has been decommissioned as of this year - in case you were trying to get into the wrong pod.