You have to click on one of the boxes with price tags below to select your tier and start a PayPal dialog.
The way they have it set up is a tad confusing.. .Most people are going to instinctively click on the big yellow pledge button and then you're greeted with a wall of text. it does say click on the image of the tier you want at the bottom, but man, they should probably design it so you see the clickable images rather than wall of text...
Anyway, I pledged at the $20 level as that's all i can really spare and it gives me everything I want (the game). Hope lots of classic cRPG can drop a 20 for this.