Ok.  Let's say you download devnp-msk.so to
the directory /tmp.  Start a terminal and get
a root command line, with a "#" prompt.  Then
do the following:

 # chmod 777 /tmp/devnp-msk.so
 # mount -T io-pkt -o verbose /tmp/devnp-msk.so

Note that you could omit the "-o verbose" - that
causes the driver to put diagnostic information 
in the "sloginfo" output.

Another way:

 # slay io-pkt-v4-hc
 # io-pkt-v4-hc -d /tmp/devnp-msk.so verbose

Now, assuming the driver doesn't give you an
error in the _attach routine - called when you
mount it - it's time to call the _init routine,
either by running dhcp.client or statically
assigning an IP address as such:

# ifconfig en0 10.42.110.5

Again, the sloginfo output will be useful, as 
will the nicinfo output, and the "ifconfig -v en0"
output.  Hopefully you should see a link speed/duplex
being detected, then start to see packets being 
received - if that happens, pretty much everything 
should work!

--
aboyd