I just realized that I could not use target=”_blank” in the WordPress MU editor. So I search in WordPress MU forums http://mu.wordpress.org/forums/ for a solution.
The answer is in this thread…
[click here to open in a new window]
Allowable HTML Tags in WordPress Editor
To fix the problem, you’ll need to edit this file wp-includes/kses.php as shown below.
Now, you can override the default filter and add in whatever HTML tags you want. I have also added “class” to the “img” tag and “style” to “span” tag