I've sent off an email to support but thought I'd setup my forum account and test the waters and see if there was some simple trick to solving this problem that someone here might be aware of...
After buying and downloading and playing Episode 1 I went ahead and upgraded to the rest of Season 1 of Sam & Max. All went fine except Episode 3's green download button does not appear and instead I get "processing serial number or unlock code", so I have not been able to download this episode yet. Does anyone have the answer on this? If not I'm sure Support will fix it soon enough.
I look forward to these games and more because I am so tired of hack & slash having come from pitifully replaying missions over and over and over again in Guild Wars until I gave up and playing Gothic 3, Knights of the Nine, Shivering Isles and S.T.A.L.K.E.R. I almost forgot my Adventure roots.