FreePlay has released a new version of the PSP Slim firmware 3.60 installer that includes fixed instructions as well as an updated program. Here's an explanation straight from FreePlay:

The previous instructions were a bit off. They didn't take into account that the IPL taken straight from a NAND dump would contain 16 spare bytes after every 512 bytes, so I wrote a program that strips those bytes out. The fixed instructions and the program are attached here.

Remember to check out the readme included in the download before you install.