Hi spindy,
Since you are unable to see the "calculated shipping" option while creating your listings, I suspect you must be posting to one of the non-US eBay sites. We currently only support the calculated shipping option for the eBay US website, but we will most likely be adding support for calculated shipping on international eBay sites with a future site update.
For the time being, the only way to offer calculated shipping on listings posted to non-US eBay sites that are listed through Auctiva would be to post them with a different shipping option and then revise them through eBay to reflect calculated shipping once they've been posted. I realize this is less than ideal, to say the least, but I figured I'd throw it out there.
If you have any additional questions or concerns regarding your Auctiva account, please feel free to contact our customer support team by completing the web form on the following page of our site:
http://www.auctiva.com/help/requesthelp.aspx-Mike