Managed Office add-ins traditionally have been able to run in the targeted application (the version of the application whose PIAs the add-in references) and in later versions of the application. Therefore, if you need to create a single VSTO add-in that can be run in multiple versions of an application, the typical guidance is to develop the add-in by using a project template for the oldest version of Office that you want to support. For example, if your add-in needs to work with Office 2003 and Office 2007, you should create an Excel 2003 add-in (by using VSTO 2005 SE with Visual Studio 2005, or by using Visual Studio 2008). However, this strategy has several inconveniences: If you want to test/debug/run the add-in on your development computer, you must have the earliest version of Office you are targeting installed

Go here to read the rest: 
Creating an Add-in for Office 2007 and Office 2010 that “Lights Up” on Office 2010 (McLean Schofield)