I had the same problem, I've dowgraded to VS 2005, installed VSTO then followed the deployment instructions on the below article.
http://msdn.microsoft.com/en-us/library/aa537173(office.11).aspxIMO, Microsoft has been terrible in giving good directions for VS 2008, the documentation should have been included in VS 2008. If you find the directions somewhere and the possibly needed Security and Manifest projects for VS 2008, let us know, too...