The Automatic network environment normally is set to DHCP. So you don't have to create a new network environment.
To check the settings of Automatic or to create a new network environment please press Network Prefenrences....
Create a new network environment and call it for example "MILan".
Check the settings and select DHCP mode. If you have to change the settings don't forget to
apply it.
Select WLAN Connection
Click on the Airport symbol in the status bar and select MILan in the menu.
How to verify the WLAN connection
Open a terminal session. Enter ifconfig. Check the output for the IP address of en1 and ppp0. The adapters should have IP addresses from the following ranges en1172.45.xxx.xxx and ppp0160.45.11x.xxx.