In case of MAC OS, you will find the AIRPORT icon at the right hand top corner. If you click On it, it will try to connect to the wireless network. If you network is secured then it will ask you for the wireless network key or password. Type the key and it will connect you to the network.
If you get error joining the network and if the wireless security type is WEP, then try to connect to the wireless network again. It will ask you for the network key / password again. Now this time enter $ sign before entering the network key. It will connect you.



Still Not able to connect to the wireless network ?