I cannot help any further other than to suggest, it seems that you may need to do one of four things ...
* switch to Visio
* switch to something else
http://list.ly/list/Ac-microsoft-vis...es-on-mac-os-x
http://graphicssoft.about.com/od/dia..._Macintosh.htm
* find and download the images you want, then paste them in to OmniGraffle appropriately
* create your own images in OmniGraffle
--
JJW