Can I connect iMac to wifi, new AirPort card, DLink router? Help pls!

Sorry to be thick.

I've been running two PCs, a netbook, iPod Touch and iPad happily on wifi router - Virgin cable broadband and their installed modem and DLink router - with no issues.

I now have a refurbed iMac 20" with OSX10.5.8 which connects brilliantly through ethernet cable, but it means wires all over the house and doors can't shut etc.

Same guy who sold me the iMac has sold me an AirPort Extreme card which I've installed.

Please treat me like an idiot - the card is 'seen' in the network box, is turned on, but greyed out at the top of screen and can't find any wifi networks to join.

Is it not possible without a AirPort base station as well as DLink router? Is it because the wifi is WPA2 secure? Is it because the card is wrong type?

I get this in info section:

AirPort Card Information:

Wireless Card Type: AirPort Extreme (0x14E4, 0x4E)
Wireless Card Locale: Unknown
Wireless Card Firmware Version: Broadcom BCM43xx 1.0 (4.170.25.8)
Current Wireless Network: wireless network not available

Please help - I've spent so much time on this, it's probably really simple, something's just wrong in the hardware or software. I don't want to lose the connection to the other devices (life wouldn't be worth living, divorce court beckons etc) - I also tried HomePlugs, but they wouldn't connect either. This could be because of electrical circuit wiring issues, as the BluRay wouldn't either - on the other hand, it did have a message about not connecting automatically and my settings were wrong.

Thanks for your patience.