Probably now the installed version is always started.
I do it as following (so that modifications are applied immediately, and you are able to debug / set breakpoints / step through code / etc)
In the Addin's project settings, set the "Register for COM Interop" checkbox.
Also set to start Visio as executable (see below), so the VS does not complain that it cannot start a DLL, but just start Visio.
Rebuild. Run (F5).
Then VS will register the debug version, and Visio will load it instead of the installed one.
Note that you'll need to restart Visual Studio as Administrator do do that:


When you are done debugging, build the installer.