My laptop was stolen about 2 months ago. I have the MAC address.
I have contacted the police, but they said they could not find the laptop if it is behind a ADSL modem or any NAT.
Is there any way I can find it?
Unfortunately, the police are right. Your MAC address is, in fact, a unique identifier of your laptop's networking hardware, but it is not transmitted any further than your modem or NAT router. Media Access Control (MAC) happens at the link layer (of the Internet Protocol Suite) and not at the Internet layer, thus this information will not be transmitted outside your local network.
This means you'll only be able to identify the laptop once you actually see it – and MAC addresses can be spoofed as well, so you couldn't even be sure about that.
If the laptop is in physical proximity, you can listen to the wireless signals with aircrack and find the laptop