I repartitioned my Boot Camp partition with CampTune and everything went smoothly. Once your Mac boots up with the disc you just follow the on screen steps. Very straight forward.
You will need to burn the .iso image you downloaded to CD/DVD via the /Applications/Utilities/DiskUtitlity application.
Once you have burned the image reboot your Mac and hold down the 'c' key. This will allow your machine to boot from the disk you just burned. From there you should be able to modify your Bootcamp partitions.
If there is any documentation on the DVD/CD you burned from the .iso, I would recommend reading it before doing anything. Also, it is strongly recommended that you back up any critical files before attempting to alter or resize partitions on your drive.