This has been discussed in the WordPress support forums http://wordpress.org/support/topic/62988.
All you have to do is, as mentioned by Felix, to install the Kimili Flash Embed plugin, and then make an embed tag which references the imageData.xml. Size and background color of the viewer can be adjusted by the embed tag itself. Here's an example posted by slambert
[kml_flashembed movie="/simpleviewer/gallery1/viewer.swf" height="500" width="500" quality="best" bgcolor="#FFFFFF" fvars="xmlDataPath = /simpleviewer/gallery1/imageData.xml" /]
You will have to modify imageData.xml, specifically the second line pertaining to the viewer's navigation bar attributes as well as the frame colors to fit your layout
<SIMPLEVIEWER_DATA maxImageDimension="358" textColor="0x000000" frameColor="0x000000" bgColor="0xFFFFFF" frameWidth="2" stagePadding="20" thumbnailColumns="2" thumbnailRows="5" navPosition="right" navDirection="LTR" title="Gallery 1" imagePath="/simpleviewer/gallery1/images/" thumbPath="/simpleviewer/gallery1/mcbeanshow/thumbs/">
This should seamlessly integrate SimpleViewer with most WordPress themes.
There are some (weird) caveats though. For some reason specific directory names do not work for xmlDataPath? i have experienced this on my site at www.virtual-toilet.com and have had to create separate directories to accomodate this... Otherwise just follow the above link for more instructions and you should be up and running in less than 1 hour.