Intel Mac :: Cannot Set Up IMessage To It

I have an iMac 24 in 8,1 Core 2 Duo 2008. I have it connected by ethernet to my router/modem which is connected to my fixed phone line for internet connection. The router has wi-fi activated [802.11 a/b/g/n] and I access the internet through it to and from my iPad 2 and my iPhone 4. None of these connections have any problems. I have not set up/activated iCloud.�

I have no problem sending iMessages between my iPhone and my iPad via my router's wi-fi.�I could [and often do] send myself an email from my iPad to my iMac with data but I wish to activate the iMessage process.�But I can not send iMessage to my iMac from my iPad. Presumably I have to activate my iMac's wi-fi and this I can not do. I click on the wi-fi icon in the menu bar which brings up 'wi-fi not configure' [greyed out] and 'Open Network preferences'. When I do open network preferences I am told wi-fi is inactive. And it seems there is nothing I can do to activate it. I have been through all the windows for network preferences.