Dell have a history of using different components in the same model of laptop. Some Dell E6400’s have Intel wireless cards and others Broadcom (BCM43xx chip).
Unfortunately Ubuntu and its derivatives such as Mint do not include Broadcom wireless drivers as standard. However its quite simple to get wireless working.
1. Open Terminal and enter the following to check your laptop is using a Broadcom device.
lspci | grep Network
2. Open Software Center, search for and install the following packages
3. Reboot, if wireless is still not working you will need to install kernel driver by entering the following in Terminal
sudo apt-get install bcmwl-kernel-source