1. Click on the "File" tab at the upper-left corner of the Excel Window.
2. Click on "Options" near the bottom of the menu.
3. Click on "Add-Ins" near the bottom of the options menu.
4. Click the "Go" button near the bottom of the Add-Ins window. This button is to the right of the label and list item which should read, "Manage" and "Excel add-Ins" respectively.
5. Check the "Solver Add-in" box and click the "OK" button.
If the Solver tool is not available then you'll have to Google "Load Excel Solver Tool" to find a site that will enable you to download this tool.