How do I extract ROMS with an NDS extension?

Hello,I am trying to download DS games to my memory card, so that I can use the memory card in an adaptor which I purchased this week for my DS. I have tried using the "download" function and saving the games to my memory card. It automatically saves them with a .RAR extension and I think I need an .NDS extension. Can anyone tell me how to do this? The .RAR files are definitely being saved onto the memory card, but when I put the memory card into the adaptor, and then put that into my DS, it is not recognizing those files. We are going on vacation tomorrow and I desperately need a bunch of games for the long car ride! Thanks!

.rar files are archives much like zip files; the .nds files are inside the .rars, and you need winrar from www.rarlabs.com to extract them.

Great - thanks! I just checked out the tutorials as well and there are detailed instructions on how to extract the game file from the .rar file. I tried it and it worked - awesome.