Go to the Roguelikes for PalmOS project page on SourceForge. Download the "iLarn module" (it should be listed in the File Releases section).
I've used prc-tools 2.0. I'm running Linux.
SDK - The current version of iLarn uses SDK 3.5 or higher. (Or, in the unlikely event that you want to compile for PalmOS 2.0, SDK 2; to compile for OS 2.0 you will also need to change which definition of CFLAGS is commented-out in the Makefile.) Prior to iLarn 0.25 I used SDK 3.1 (the upgrade involved some header file changes.)
multigen - Since iLarn is large, it has "multiple code resources" and uses multigen. You might want to read about that in the prc-tools documentation.
emulator - The safest place to test software is in a Palm emulator. You will need a ROM image to use it.