.rar files are like .zip files, you need to extract them first before you put them on your gp32. Get a program called WinRAR or another program that can handle rars.
WinRAR is technically a trial version, but all it does is give an annoying pop-up message after the trial expires, and you can just close it. I use WinRAR and I love it. Especially batch compression and decompression.