I've a secong hand white macbook, the model is 4,1 early 2008. My Wi-Fi keeps disconecting and it says there's no hardware installed, I need to restart for it to get back to normal. I think there's maybe something wrong with the airport card so I'd like to run an AHT, the thing is I don't have the original disks so I found a supposed AHT on the internet but I don't know how to install it. There's a .dmg file but when I restart and press D nothing happens.