The plugin uses the WordPress default styles for galleries to show the thumbnails in the proper way. It have to show them on the same format as the default WP galleries when inserted into a post. If you see only one column, it's because your theme has not full support for WP 2.8, as it does not have styles for WP-Galleries.
Here you have the styles I use on my Chameleon theme to show the thumbnails on columns. Paste it in your theme or plugin style.css file and test it. The columns number is set automatically depending on the thumbnail size, just remember to keep this if you update the plugin or the theme (depending on which style.css file you set it):
.gallery {
margin: auto;
margin-left: auto;
margin-right: auto;
}
.gallery-item {
float: left;
display: inline;
margin: 5px;
padding: 3px;
border: 1px solid;
text-align: center;
}
.gallery img {
border: 0;
}
.gallery-item:hover {
}
.gallery-caption {
margin-left: 0;
}
img, .gallery-item {
-moz-border-radius: 5px;
-khtml-border-radius: 5px;
-webkit-border-radius: 5px;
border-radius: 5px;
}