If I understood correctly, you think Messenger Plus! is the latest version of WLM. Messenger Plus! is an add-on for WLM, whose latest version is 2008 or 8.5. So, if you can see the Plus! buttons in the toolbar of the contact list, then Plus! was installed properly and you can enjoy its features and spice up your Messenger. Thus, there is no problem.
You know, when I fisrt installed Plus!, I thought too that it was the latest version of MSN, as I was told by a friend.
If I didn't understood correctly, then you have the right to call me newb.