On Windows, Solver may be added in by going to File (in Excel 2007 it’s the top left Windows button) > Options > Add-ins, and under the Manage drop-down choosing Excel Add-ins and pressing the ...

Excel doesn't have Solver by default. It's an add-in, so you have to load it first. Fortunately, it's already on your computer. Go to File > Options > Add-Ins.