rar files

ok new to this site but looks informative and hope someone can help,before i go loopy, ::)anyway il star by sayig hi,i recently bought my son a ds for his b'day,but when i download the games they download in rar files and when i put them on a r4 card they wont play so could someone please instruct me on what to do to get these games to play
thanks in advance

Here's a nice tutorial. :)