There are a lot of things that may prevent the add-in from loading:
You didn't include all the required prerequsites to the add-in installer.
I'd recommend checking the COM add-ins list in Outlook. Is your add-in listed there?
Microsoft Office applications can disable VSTO Add-ins that behave unexpectedly. If an application does not load your VSTO Add-in, the application might have hard disabled or soft disabled your VSTO Add-in.
Hard disabling can occur when an VSTO Add-in causes the application to close unexpectedly. It might also occur on your development computer if you stop the debugger while the Startup event handler in your VSTO Add-in is executing.
Soft disabling can occur when a VSTO Add-in produces an error that does not cause the application to unexpectedly close. For example, an application might soft disable a VSTO Add-in if it throws an unhandled exception while the Startup event handler is executing.
When you re-enable a soft-disabled VSTO Add-in, the application immediately attempts to load the VSTO Add-in. If the problem that initially caused the application to soft disable the VSTO Add-in has not been fixed, the application will soft disable the VSTO Add-in again. Read more about that in the How to: Re-enable a VSTO Add-in That Has Been Disabled article.
- Do you get any UI errors?
By default, if an VSTO Add-in attempts to manipulate the Microsoft Office user interface (UI) and fails, no error message is displayed. However, you can configure Microsoft Office applications to display messages for errors that relate to the UI. You can use these messages to help determine why a custom Ribbon does not appear, or why a Ribbon appears but no controls appear. See How to: Show Add-in User Interface Errors for more information.