I checked out those T1500 drivers and they are quite new drivers. January 08, 2010. I'm not sure if the download pack is downloading these drivers. You can do this another way by going out to Dell's site and download the XP broadcom drivers for your T5100 manually. You should only need the 32-bit version for your version of winpe.
After you download them, extract the drivers with 7zip or some extracting utility. The folder you want to pull from there is:
those should be the correct drivers.
if you still want to use the driverpack utility to add your drivers, just create a new folder where the wpedownload.exe is downloading the drivers and name it broadcom4.nic and drop those files into it.
Now add your drivers to the pe and it should work.