Dining plans are only available via package reservations made through Disney or an authorized travel service. Expedia is not one of those services -- they are generally a discounter that resells excess inventory.
A dining plan package includes resort, tickets, and dining plan. Annual passholders may book a "ticketless" package that includes just resort and dining plan, but otherwise they still have to follow package rules (such as prepayment and cancellation/change policies) if they want a dining plan.
DVC members staying on points may add a dining plan to their stay with or without a ticket purchase. This is handled through DVC and not the public reservations system.