So as you've discovered, you can download the MPLAB IDE from Microchip at
this link.
You can get the C18 compiler
here. That's the student edition, but it's good for 30 days or so, which should be enough for you.
You'll also need IFI Loader from
here. Some of the other programs there can be useful as well.
Finally, if you're planning on using this combination of software, you HAVE to use Kevin Watson's C18 3.0+ compatible code available
here.