Although not very usual, add-ins can be created for the Macros IDE too. In fact, version 6.0 of MZ-Tools integrates with the VS IDE and with its macros IDE. I will remove support for the macros IDE in future versions because I think that nobody needs/uses it and it complicates the code and the setup (specially the uninstallation), but until then, it works with both IDEs.
While the VS IDE executable is devenv.exe, the macros (VSA) IDE is vsaenv.exe. From the automation (EnvDTE) point of view, the EnvDTE.DTE class has the EnvDTE.DTE.MacrosIDE() As EnvDTE.DTE property.
Until VS 2010, each VS IDE has its own macros IDE with matching versions. However, VS 2010 (VS 10.0) actually uses the VS 2008 Macros IDE (VSA 9.0), as you can see by the look and feel (not WPF-based) and in the About window.