I'm going to assume you have already installed bitcoin-qt and that there is an existing wallet.dat in the bitcoin data directory.

1) Make sure bitcoin-qt is not running. (You can check using System Moniter)
2) Go to the bitcoin data directory (it's installed in the home directory by default - if you can't find it this link should help https://en.bitcoin.it/wiki/Data_directory)
3) Remove the current wallet.dat file and put it somewhere safe (the desktop works fine)
4) Place the wallet.dat you want to import in the .bitcoin directory
5) Start bitcoin-qt.

Note: this method allows you access to the backup but does not merge the two wallets.  If you want all your coins in the same wallet you need to send the coins from the wallet you don't want to use to an address in the wallet you do want to use.