I've been getting this problem as well.   Currently on WLM 8.01 (unskinned) and the latest Messenger Plus.   
No icon for Plus! or a Script button appear on the tool bar.
As a side note:
1 script has been imported and is running, the checkbox in preferences is ticked.
However, frustratingly no Plus! button nor Script button appear on WLM 
 
Any help appreciated.