Where do I find the Minecraft tribute?

Solution 1:

I found it in the area "The Sundered Battlefield". There is a small instance called 'Notch's Mine', which contains a few creepers.

It is worth keeping in mind that since Torchlight 2's areas are put together randomly, it could be anywhere.

Solution 2:

To be more specific, it'll always be in The Sundered Battlefield, in the Grave Deep section. It won't always show up though, but if it is gonna show up, that's where it'll be. So if you don't see it, create an Internet game and choose Reroll World, go find the Grave Deep section in Sundered Battlefield, rinse, repeat until you see it. It'll be a regular instance entrance like any other on the map.