Several users have reported an issue in which Mac OS X 10.6.x (Snow Leopard) systems cannot connect to Wi-Fi networks to which computers running Mac OS X 10.5.x, iOS devices and Windows systems have no problems connecting.
Turn off Bluetooth
Oddly, some users have reported that simply turning off Bluetooth resolves this issue. To do so, Simply access the Bluetooth menubar item and select “Turn Bluetooth Off” or open System Preferences from the Apple menu, click Bluetooth, and uncheck the box “On.”
Other potential fixes for Wi-Fi/AirPort issues under Snow Leopard include:
Update router firmware. Check your router manufacturer’s website, and download and install any new updates for your wireless router.
Reset SMC. Some users have reported that resetting the Mac’s SMC (system management controller) at least temporarily resolves this issue.
Reset router settings. Try resetting your wireless router’s settings. For AirPort base stations, this can be accomplished using the AirPort Utility. For third-party routers, this can generally be accomplished by either holding down a button on the back of the unit (see your unit’s manual for details) or logging into your router’s configuration page by opening a browser window and entering the address 192.168.1.1 (you may need to enter the username: admin and password: password1) then using the reset function.
Change or turn off wireless security You may want to try toggling the settings on your wireless router, switching from WPA to WEP or vice versa, or, as a last resort, turning wireless security off altogether. For AirPort routers, this can be accomplished with the AirPort Admin Utility. For other routers, this can usually be accomplished by accessing the router’s configuration page — open a browser and enter the address 192.168.1.1.