Here's a copy of my standard instructions to students:
Excel's Solver
If the Solver command doesn't appear on the Tools menu, choose the Add-Ins
command from the Tools menu. In the Add-Ins Available list box, check the
box for Solver.
If Solver doesn't appear in the Add-Ins Available list box, you may need to
add the Solver through a custom installation using the Microsoft
Excel/Office CD Setup program.
The Solver add-in files are not available for download. They are available
only from the Excel/Office CD.