Maybe there is a simple answer but I am stumped...

Why do I get host unreachable for an IP address that I can ping with the ping command from a command prompt?

There are a few possibilities.

First off, you should know that PingPlotter is more a peer of the command line utility "TRACERT", not PING. What kind of results do you see with TRACERT?

Next, you might have a look at this post:

http://www.nessoft.com/forums/showflat.php?Cat=&Board=Support&Number=529

Which discusses a situation that has some similarities to yours.

You might also try changing PingPlotter to ping the final destination only to see if that changes the results (View menu, Ignore First Hops -> Ping Final Hop Only).

Let us know what results you get with TRACERT, what results you get with Ping Final Hop Only, and if you try any of the ideas in the post referenced above. We'll do what we can to get to the bottom of this.