Hello juujuu,
If your using the Windows wireless to connect to the router, you will see (if his/her router is not hidden) it appear in the list of connections:
^ You see this network has security enabled. When you press [Connect], you will be asked for the WEP key you listed above.
If you do not see the router in this list, then check to see if the router is broadcasting it's SSID from the router admin pages. Signal strength is also a factor here, so even if the signal does reach you, ensure that the strength is at least 2 (sometimes 3) bars.