Provided you know the network name and password, connecting an iMac to a Wi-Fi network should take only a few seconds. Once you connect the first time, the iMac remembers the network so you won't have to log in again the next time. If your iMac doesn't see the network, or if you don't seem to have an option for Wi-Fi, you may need to do some basic troubleshooting. Aside from damaged equipment, you should be able to connect within just a few minutes.
To connect to a Wi-Fi network from an iMac, click the "Wi-Fi" icon in the menu bar located in the upper right corner of the screen, click "Turn Wi-Fi On" if needed and then choose from the list of available networks. If the icon isn't visible, launch "System Preferences" from the Apple menu, click the "Network" icon and then click "Wi-Fi." If the button on the right side of the window says "Turn Wi-Fi On," click it. You can then click the "Network Name" menu and select the Wi-Fi network. If there are a lot of Wi-Fi networks in the area, it may take five or ten seconds for all of them to appear. Enter your password when prompted, and you will be connected.
Verifying Your Wi-Fi Adapter
If there isn't a Wi-Fi option when you open Network Preferences, click the "+" button in the bottom right corner to add Wi-Fi, then click the "Turn Wi-Fi On" button. If this doesn't work, verify that your iMac has an AirPort Wi-Fi adapter by selecting "About This Mac" from the Apple menu, clicking "More Info" and then "System Report." If your iMac has a working adapter, its details will be listed when you click "Wi-Fi" in the Network section of the left menu. The adapter is listed in the Interface section. If there isn't one listed, make sure Mac OS X is updated and, if necessary, contact an authorized Apple technician for repair.
If the Wi-Fi adapter seems to be working correctly but you can't join a network, verify that you have the right password. If the network isn't visible, try moving the iMac or the Wi-Fi router so they are in the same room without any large objects between them. As long as you know the network name, or SSID, the password and the encryption security the router uses, you should be able to connect by selecting "Join Other Network" located beneath the list of networks. Most home networks use WPA2 Personal encryption. You should never join a network without WPA or WPA2 security or you risk having your network and your data hacked by anyone nearby.
As with any other device, an iMac can't join a Wi-Fi network without permission. Ask the person who set it up if access is limited to specific computers. If so, ask him to allow the iMac access. This is usually done by entering the computer's Media Access Card, or MAC, address in the router's admin panel. In the case of any Apple computer, the MAC address you need is listed beneath the Wi-Fi adapter's name in its System Report, accessible from About This Mac. This is a unique number for your computer, such as "a8:86:de:a9:87:79."