SharePoint 2010 Troubleshooting: Cannot open the Excel files
Issue
Users cannot open excel files in SharePoint 2010 from the Library and get an error "This workbook cannot be opened because it is not stored in an Excel Services Application trusted location. To create an Excel Services Application trusted location, contact your system administrator"
http://3.bp.blogspot.com/-YrLIo2px4AY/UntJ9Mi0-nI/AAAAAAAAAhE/EFI7kvkGtQE/s1600/55.bmp
In previous versions of SharePoint, what users do is they contact SP admins and ask them to add the library to a trusted file location (as error says) but in SP 2010 it's not the case
Cause
By default the files are designed to open in a browser. So it is checking for a trusted location.
Resolution
Follow the below steps to resolve the issue.
Go to the Library
At the top click Library and go to Library settings
Under General Settings you will find Advanced Settings
Over there you will find the option which by default is set to open in browser
You need to set it to Open in Client and there you go
Click OK and you are done