What are you exactly asking for?
Seeing your thread title you ask for the command /bigkiss.
That command doesn't exist.
The command /skiss does exist though and will show something like "XxxxXxxxX" and play a kissing sound.
There was however, in Plus!3, a
default quicktext with /bigkiss as alias. This default quicktext would have been only added when you didn't already had a quicktext (thus it was added as an example).
In Messenger Plus!Live, this example quicktext isn't added by default. But of course you could create it yourself by going to the Plus! preferences:
Plus! > Preferences > Conversations > Quick Texts > Add...
Fast Send Shortcut: /bigkiss
Messages: /me grabs [c=39](!N)[/c] gets real close and plants a [c=38]DEEP, WET, passionate kiss (K)(K)(K).[/c] [c=39][b]WOW!!!!![/b] that was great![/c] (L)
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Seeing your actual post, you're asking for where to download Messenger Plus! Live.
If you visit the official website (
http://www.msgplus.net) you'll automatically be directed to the site of Plus!Live:
http://www.msgpluslive.net/.
But, you can also even simply press the "Download" link on top of this forum too...
If you want to visit the official website for Messenger Plus! 3 you should go to
http://old.msgplus.net/.
EDIT: hmmm, I see where the confusion comes from, you forgot the word "in" before "Messenger Plus" 
...
/ 