There is an official documentation of how to install word plugins.

https://learn.microsoft.com/en-us/office/dev/add-ins/publish/publish

You can choose any method you prefer to install your plugin.

Other than that, check that you are using MessageBox, from winapi, correctly.

You marking some functions as "C". Are you sure that MessageBox is a C, not C++ function?

int MessageBox( [in, optional] HWND hWnd, [in, optional] LPCTSTR lpText, [in, optional] LPCTSTR lpCaption, [in] UINT uType );

uTyPe parameter is the only non-optional parameter to the function. You pass zero to the parameter. Is this expected?

Turn on warnings for your compiler. The one that i see straight away, is that function parameters is unused.