If you want to know whether Plus's setting for it is currently enabled, try this piece of code:
code:
var objShell = new ActiveXObject("WScript.Shell");
var boolTabbedChatsEnabled = false;
if(Messenger.MyStatus > 1)
boolTabbedChatsEnabled = objShell.RegRead("HKCU\\Software\\Patchou\\Messenger Plus! Live\\" + Messenger.MyEmail + "\\Preferences\\TabChatAuto") == 1;
// now use like
// if(boolTabbedChatsEnabled) { ... }