First, make sure the wifi is enabled using the hardware switch Fn+2 key.
Second, click the network icon in the tray (at the top of the Ubuntu screen), and either
a) click the SSID of your wireless network, or
b) if your wifi network doesn't broadcast it's SSID, enter the specific SSID of your wireless network.
Are you using secure wifi? If so, it should ask you for the specifics of your configuration.
Obsidian Mini 9 >
2gb ram, 16gb ssd, bluetooth, 1.3mp webcam, us-intl kbd Internal Mods >
usb hub, 16gb flash drive, 8gb flash drive, added webcam, changed kbd Software Mods >
Dellbuntu 8.04.1 | Gentoo 2008.0