From the Serive Manual - looks like you can buy a mini pci card spearately, although they mention they dont support non-dell brands...:
If you ordered a wireless Mini-Card with your computer, the card is already installed.
Your computer supports two Mini-Card slots:
Two Full Mini-Card slots - for WLAN and Mobile Broadband or WWAN
NOTE: Depending on the configuration of the computer when it was sold, the Mini-Card slots may not have Mini-Cards installed in them.
Your computer supports two types of wireless Mini-Cards:
Wireless Local Area Network (WLAN)
Mobile Broadband or Wireless Wide Area Network (WWAN)