Go to File > Options. (In Excel 2007, click the Office Button and then Excel Options.)
Go to the Add-Ins section.
Look to the bottom of the window and next to where it says Manage: make sure you have Excel Add-Ins selected. Click the Go... button to the right of the drop-down menu.
In the window that opens, put a check mark next to Solver Add-in and click OK. (If you don't see the Solver Add-in in this window, you will have to click Browse and locate the Solver file on your computer in order to use it.)
Go back to Excel and go to the Data tab and look to the right and you should now see the Solver button.
Developer Tab Add-In Install
If you have the Developer tab visible in Excel, you can easily install add-ins from there.
Go to the Developer tab and click the Add-Ins button.