Looks like the plugin developer may have abandoned support so I'll share what I've learned so some of you may benefit.
This plugin makes columns work using a series of
<div> tags which can be very badly placed if you don't get your line breaks right in your post content. I suggest that a column closing tag and the following opening tag appear on the same line in you post content. For example:
[/one_half][one_half_last padding="0 0 0 10px"]
This avoids alignment issues and
<div> tags appearing where they shouldn't.
Unfortunately this plugin will inject a final
<div class='clear_column'></div> tag inside your last
<p> tag which is poor semantic HTML.