First you need to unzip those files you just downloaded. Second, you need to copy the "boot.img" and "system.img" (not the *.zip) to the "/SDCARD" folder on your SDCARD. I had to use WIFI FILE Explorer in Nookie Froyo cuz I messed up my nook so bad, i couldn't copy these files by turning on the USB Storage through the regular method. It took me days to figure this out cuz i'm kinda slow.
I hope you installed the Android-SDK on your computer already. If so, then go to the folder named "\platform-tools" on your PC - this is where ADB.EXE is located. Connect your nook and type "ADB DEVICES" just to make sure your nook is connected to the computer. It'll show up and tell you it is or not. If it is connected then do the following:
1. Type "ADB SHELL" then you'll see a "#" symbol after you hit enter.
2. Type "dd if=/sdcard/boot.img of=/dev/block/mmcblk0p1" just the way it is written by the original poster (minus the "" marks)
at this point, it usually wont say anything. Just wait a few minutes and something will pop up. In my case, it says:
"477033984 bytes <454.9MB> copied, 138.757660 seconds, 3.3MB/s"
3. Type the second command next to the "#" sign "dd if=/sdcard/system.img of=/dev/block/mmcblk0p5"
wait another few minutes til the next confirmation pops up.
4. turn off nook, remove SD card
5. Hold the N button and the Power button at the same time and it'll ask you to reset the nook to factory settings.
I hope this helps you ....