The Informant Sync service does not have a web interface so there isn't anything to login to online. The two things that you can do online with regards to your sync account is to reset the password or renew* in addition to the initial creation of your account.
* You can only renew your subscription to the sync service after the account has expired. Your data will not be lost you will just not be able to sync until you renew.
Renew link https://pocketinformantonline.appspot.com/login