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