Well, you have to relist directly through eBay to get your relist credit. So in order to do that you can revise your item at Auctiva, save it, tick the box next to the listing in Saved Listings, then hit the "Get HTML" button at the top of the page.
You then take that HTML and plug it into the description box on eBay's relist page.
Although, I'm not sure if you mean you want to revise a closed listing or an active listing. Either way, though, the above should work.
Hope that helps.