I also ran into the same situation when attempting to pxe boot and install ESXi5.0.0, the simplest thing is to just bypass weasel altogether and setup networking manually at the end of the install via commenting out the 'kernelopt=runweasel' line in the boot.cfg file just add a # at the beginning of the line.
In my case I used the method prescribed by PatrickD to see which interfaces were being seen by weasel and it was only seeing my pci add-on nics and not my onboard nics which is what I pxebooted from.