Dell Optiplex 620's don't have built-in WiFi. It could be ordered as an option from Dell at the time of purchase. If you have the original Dell 1450 wireless adapter, visit the dell support site for the Opti 620's @ http://support.dell.com/support/downloads/driverslist.aspx?c=us&cs=555&l=en&s=biz&ServiceTag=&SystemID=PLX_GX620&os=WW1&osl=en&catid=&impid=
If you don't have a Dell 1450 WiFi adapter, then go out and buy a PCI or USB wireless adapter and install it. Depending on what type of 620 you have, you might need a low-profile PCI card (the towers use regular cards, the SFF, USFF, and Desktop chassis uses a low-profile card). If you have a SFF, USFF, or desktop, i'd recommend just getting a USB external WiFi adapter.