what happens when you run the command from the command line?
/a = the name of the application, and should be something like "Solarwinds" or "Orion"
what happens when you run the command from the command-line on the alert server?
C:\Program Files (x86)\Growl for Windows\growlnotify.com /a:"Orion" /t:"A Network Node is Down!" /host:10.154.100.1 /pass:"mypass" "Test"
this should popup somehting that says 'Test' -- if it does not then you need to fix that
if you want an icon then you can use (one time only needed)
C:\Program Files (x86)\Growl for Windows\growlnotify.com /a:"Orion" /ai:"someicon.ico" /t:"A Network Node is Down!" /host:10.154.100.1 /pass:"mypass" "Test"
which will set the icon used for "Orion" alerts