Well there are a few possible explanations. Without seeing your code I don't know what scenario is going to be yours but here are two potential ones.
OnEvent_ChatWndSendMessage () you need to return '';. If you do not the command gets passed onto the Messenger Plus! command interpreter and therefore because it is not a valid Plus! command it provides the message.
Secondly and least likely is that the command is not defined OnGetScriptCommands.
Post your code and we can have a better look.
This post was edited on 09-04-2009 at 12:45 PM by matty.