1) Make sure you have the latest Messenger Plus! (
http://www.msgplus.net)
2) Make sure the icon is enabled:
Plus! > Preferences > Instant Messages > Enhance the toolbars with Messenger Plus! features.
3) Make enough room in the typing area in a conversation so the toolbar can be shown.
4) Some skins are not compatible with Messenger Plus!'s toolbar.
(PS: this question was asked before, a
forum search can solve many things
)