If you don't receive it in the next day or two then i'd suggest that you give the do store people a call. Use the toll free number available on the do store site and not the do gear offer phone number.
I called them last wednesday. They said that the offer is handled by a separate section and that they'd send an internal email to that section asking them to contact me reg. status of my ticket. Lo presto. 6 days (of which 2 were holidays) later my voucher code has arrived.
It could just be a coincidence that the voucher arrived soon after I made that call. May be not....
So, as I said earlier, you could give them a call if you don't receive the voucher in the next few days. It might help...