There definitely seems to be an issue with the return process. I've had no labels etc, and DHL arrived yesterday to pick up the DTK with no notification beforehand so I wasn't actually in. I spoke to the Apple Support to understand how I rebook a pickup and they didn't know. They just pointed me to the return page which just says DHL will contact us.

DHL have come back today for me and took the DTK quite happily without labels!

Have you tried the link at the top of this forum? The one that's called something like "return instructions/promo"? I haven't heard anything since returning my DTK to Apple around the same date as you, but yesterday followed that link and the "apply promo" button was there. Worked perfectly for me - have bought a new Mini and applied the promo to that using the link on that page and it arrived today. Weird that I've had absolutely no confirmation from Apple though that they've received my DTK back or I have a promo. The whole thing is a bit of a mess really for a company as slick as Apple usually is.