Cant play the games that we have down loaded for Nitendo DS

Can any one explain what i need to do.
I have down loaded some games to a file on the computor, and then tranferd to micro sd card for the children to play but nothing happens.
I ahve spocen to some one and they tell me its because the file is compressed??
Need to find file that opens to play, where do i find what do i have to do.
Many thanks to all that help.Hapy new year to all

You need to download winrar from here - http://www.rarlab.com/rar/wrar391.exerun the installer and once it is done you just right-click on the .rar file and hit the "Extract here" button.it will extract to a .nds file and you take that and put it onto your micro sd card.make sure that you have the firmware on the micro sd card too. The files that make the card work.

What card are you using?, from your box or card..