my solver paced in Addins folder is load automatically. How I know
that? Go to tools>addins>select>navigate to Office 2004>Office>Add-Ins
and pick the solver.xla and Excel will tell you. That's why it's not
listed. When I want to use the solver I go Tools>Solver... Directly
in the tools menu (no in the adins).
Did you know, you can update your office to 11.3.4?
Just do help>check for updates or follow instruction in here