You will need to get Airdecap, you only have the .cap files so you need Airdecap to decrypt those capture files...
http://wirelessdefence.org/Contents/...k_airdecap.htm
Read up more about it at that link. I have a aircrack and airdecap and a airodump for windows if you want? Very easy user interface. Cain and Abel can also do that but I am not sure how as I don't use Cain for that purpose, Ethereal might help you with that too.