How to add a file in excel trusted file locations:
1. On the SharePoint Central Administration Web site home page
2. In the Application Management section,
3. Click Manage service applications.
4. On the Manage service applications page
5. Click the Excel Services service application that you want to configure.
6. On the Manage Excel Services Application page
7. Click Trusted File Locations.
8. On the Excel Services Application Trusted File Locations page,
9. Click Add Trusted File Location.
