You have to find someone in your organization who has permissions to add users in PowerSource. They will need to add you which will result in an email invitation to you to register.
In PowerSource > Support tab > Case Management, if you filter by
Show = My Cases, Status = Any Status and Created = Anytime
and STILL do not find your cases, I would submit a Support Case to find out what happened to your cases on the old Support site. They should all have transferred to the new site with the creator email intact.