This is a common question with VSTO 3.0 add-ins.  Office only allows VSTO add-ins to be registered under HKCU in the registry which means they can only be installed for the user that runs your add-in setup on their account, not available to all users on a machine after you run the installer under one account.

There are some reasons for this behavior--in general it is better to not install for all users unless you absolutely have to as it requires an elevation of privilege to write to HKLM and it assumes all accounts that are on a machine need that add-in which may not be a reasonable default.  But you will have cases where you need to get around this limitaiton.

Fortunately, Misha Shneerson has blogged about a workaround to get over this issue.  See his two articles on the topic:

Deploying your VSTO Add-In to All Users (Part I)

Deploying your VSTO Add-In to All Users (Part II)