How do I uncompress rar files?

I've downloaded Trauma Center 2 and extracted the nds file from the rar file but when I try to play it, it says on the DS screen 'could not access the data' what should I do with it?

If you have extracted it correctly using winrar, your problem is most likely an out of date firmware on your flashcart.

Thanks, but how do I update my firmware?

Go to http://www.linfoxdomain.com/ds/, and download the latest firmware for your flashcart.

are you using a flashcart or an emulator?(you never said) ...you might also be using the wrong save type try a different one (if you are using an emulator)

I'm assume DS, since the person said ds screen, and they said where do I update my firmware.