The image you downloaded probably didn't have the network driver included. The easiest solution would be to get the ESXi 5.1 OEM iso from Dell, if they offer one.
The other option would be to download the offline bundle package for ESXi 5.1 from VMware, and use the PowerCLI Image Builder to inject the driver for your network card and export the bundle to a bootable ISO.
What network controller is being used in your server? Have you checked the hardware compability list to see if it is supported?