First of all, if these add-ins are .NET assemblies, you can always get their path using Assembly.GetExecutingAssembly().Location, among a few similar ways. To set the path in the install, though, set the registry value to [INSTALLDIR] (more than likely, if all your assemblies go into the same installation directory). There are much better ways but the VS.NET Windows Installer project doesn't make them possible without using a tool like Orca (from the Windows Installer SDK) and modifying the MSI package after it's compiled, which also requires some knowledge about MSI packages.
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