Where should I put the PIE folder that came with my stack?
PIE is Code for a Internet Explorer, that allows IE to recognize and display a number of CSS3 properties. Joe includes this folder with many of his stacks, you just need to look on the disk image of the stack. Find the folder called Resources and double click on it to open it.
In the resources folder is a folder called PIE, take that folder and put it at the root of your webserver.
Now your site should have the effects it needs in IE 8 and 9.
Now go forth and make your websites great!!