Every device, and the various internet adapters have their unique MAC address. This means, if you have a new computer, or if you used cable before but now you want to use the Wi-Fi, you have to find the apropriate MAC address belongs to that adapter, and modify your registration in this site.

You can find the adapters MAC addresses with the help of this article.

After any modification in the registration you have to wait 10 minutes, then you must restart your computer in order to get it working.

Sometimes on Windows systems the IP address is cached.
If restarting the computer doesn't work:

1. click start menu,
2. search for "cmd" and open it,
3. A black command promt appears. Enter the following: ipconfig /release
4. press ENTER,
5. Enter the following: ipconfig /renew
6. press ENTER,
7. then you can close the promt.

This will forget your IP address and request a new one.