I tried looking at my sample developer account and I can see the Customer Portal (Customize -> Customer Portal).
However, I can't see Customer Portal on a sandbox I am working on. Where do I start to get the customer portal in sandbox?
Starting from summer 13 customer portal are not available for new orgs. Your developer org must be old created before summer 13. You need to explore communities in place of customer portal. Ref: https://help.salesforce.com/apex/HTViewHelpDoc?id=customer_portal_setting_up.htm&language=nl