eSATA hard drive not detected??

I just purchased a Dell Optiplex 755 Desktop. I had it configured to have the external SATA (eSATA) connector installed. When I got the 755 I reformatted and installed Windows XP Pro. I had some trouble getting windows installed because it was not detecting the SATA controller. After some investigating i found I had to load the drivers on a floppy disk and press F6 during Windows setup to install drivers. So I got Windows up and running. Then I bought a 500GB external hard drive from Newegg and a eSATA cable from Monoprice. I hooked up the hard drive up to the 755 and turned it on. Nothing happened on the computer, no new devices were detected nothing. I thought that maybe the drive just needed to be formatted so I went under computer management then under disk management but there was no new drives. I am pretty sure I loaded all the possible drivers for my 755 from dell's drivers and downloads section on their website. No CD came with the drive, and the manual said it would just be detected by windows. I checked the Western Digital's website and no drivers there. There was a diagnostic utility that I may try later on but I doubt that would help if my computer isn't even recognizing the drive. So that leaves me thinking that either the cable is bad, which I had to buy separately from Monoprice because the drive only came with a USB 2.0 cable and no eSATA cable. Or something is missing in relation to the controller (SATA)?? I dunno I am a bit lost at this point. Oh and the 755's (at least the desktop version) has a eSATA port on the motherboard. I paid $3 extra for dell to put in this external extension. Basically it is a eSATA extension cable that plugs into the port on the mobo and takes up one of your pci lots but does not plug into the pci connector. See manual. Someone please help! Thanks