Well, I had this problem after upgrading to CS6 and removing CS4. Internet searches will find lots of threads on this but none worked until I tried this. Most registry edits deal with putting values that point to application paths. Mine were already there, and it still didn't work. Reinstalling DPP didn't work. Rebooting didn't work. However, the registry entry that "activates" this option in DPP is below:
apparently this is what DPP uses, and by adding the key "ProgID" and for CS6 the value "Photoshop.Application.60" the transfer to photoshop option is activated (at least on my machine) and works. On mine, the CLSID entry was there, but no ProgID key.
Also, here is the URL I found the most helpful figuring this out:
As always, with registry edits, proceed at your own risk.