I've no explanation as to why these ROM resident files are not on your system?? They are ROM resident files!Here is a link posted on another board in this forum to the files you are looking for:
http://groups.yahoo.com/group/MobileProHPC/files/CE 2.0 Ethernet Components/
Just because the 3COM card works with your laptop is not a gaurentee it will work on the Z50. Windows CE only supports NE2000 compatible ethernet cards.
In fact, the FAQ on Chris DeHerrea's website, states that you need to download a driver from 3COM, rename it, and then install it on your CE device. (The Z50 is NOT mentioned as one of the compatible devices for the card you are trying to use).
Good Luck.