Until recently Trine 2 and the Goblin DLC had been separate listings on Steam, however these were recently replaced by "Trine 2: Complete Story".
And instead of buying the Goblin DLC, now you have to purchase an "upgrade" For £3.49 (in the UK).
However for a very short time (until 20th June), because some people felt let-down that they couldn't get a discount on the upgrade when the Complete Story was discounted, you can email Frozenbyte to get a -75% coupon 'just for upgrade'. I just picked it up for £0.87!! Very generous of them!
Link to the post is here:
PS If you don't already own the original Trine 2, this is no use to you!