This occured to me aswell. It seems as if the MsgPlus installation doesn't install the necessary plugins to WMP in Vista (32bit). What worked for me was to deselect the script from the preferences and remove it, also physically from the scripts folder in your MsgPlus installation.
I then re-downloaded and installed the Now Playing script. Closed MSN again, opened WMP making sure the True Launch Bar Remote Control plugin was enabled (Tools > Plug-Ins >).
Run MSN again, and enjoy a working Now Playing script
