How to manage Add-ins
To install the add-in, follow the next steps:
1. On the File tab, click the Options button:
2. In the Excel Options dialog box, in the Add-Ins tab, in the Manage list, select Excel Add-Ins, and then click the Go... button:
3. In the Add-Ins dialog box, place a checkmark next to the item named Solver Add-In:
4. Click OK.
See also this tip in French: Comment gérer les compléments.