You need to add the registry entry (in your setup project) pointing to the office manifest (vsto file) into the Office registry hive. See this post describing how to do for all users or just the current logged in user. See MSDN reference for registry entries required for application-level addins.
Single User Hive
HKEY_CURRENT_USER\Software\Microsoft\Office\Word\Addins\[add-in ID]
All User Hive (x86 Office on x86 OS)
HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Office\Word\Addins\[add-in ID]
All User Hive (x86 Office on x64 OS)
HKEY_LOCAL_MACHINE\SOFTWARE\Wow6432Node\Microsoft\Office\Word\Addins\[add-in ID]
All User Hive (x64 Office on x64 OS)
HKEY_LOCAL_MACHINE\Software\Microsoft\Office\Word\Addins\[add-in ID]