When you try to change the contact what error message do you recieve?
Which way are you trying to rename the contact? By Right Clicking and selecting Messenger Plus! Extras then Rename Contact or are you trying to do it through the Preferences. If you have only tried it one of these ways please try the other way mentions.
Also but posting more then once in a row is against the forum rules. These rules can be found
here. Please take a look at the forum rules because it clearly states to not double post. Instead use the
button to the bottom right of the specific post to add more or change what you have put.
Matty