You're running on windows. Iago has only been tested on linux & OSX. We believe that Iago *should* work on Windows, but it has never been observed. Recently we switched to the scrooge compiler which was the last known impediment to getting Iago to build on Windows.
Iago has three parts, the launcher, server, and feeder. The launcher runs a shell script that starts the server and feeder. That could only work on windows if you were running in cygwin or something.
There's two things puzzling about your error message: EchoLoadTest is not a "main" class, and it should have been found. I suspect the paths in the metafile of the jar are relative and you were not launching iago from a place where these relative paths make sense.
Once you get that sorted, you'll notice that Iago generates a couple of scripts for running the server and feeder. You should translate those scripts to DOS and you should be good to go.
Good luck -- we would love to announce to the world that Iago worked on Windows.
Patches welcome! It would be awesome to modify the launcher to generate DOS flavored scripts when appropriate.