excel services sharepoint 2010 trusted location

Workbooks that you want to access must be placed in a trusted location. If they are not, calls to open the workbooks will fail.

How to add a file to trusted file location:

1.On the Start menu, click All Programs.
2.Point to Microsoft SharePoint 2010 Products, and then click SharePoint 2010 Central Administration.
3.On the SharePoint 2010 Central Administration page, under Application Management, click Manage service applications.
4.On the Manage Service Application page, click Excel Services Application.
5.On the Manage Excel Services Application page, click Trusted File Locations.
6.On the Excel Services Application Trusted File Locations page, click Add Trusted File Location.
7.On the Excel Services Application Add Trusted File Location page, in the Address text box, type the location to save your workbook—for example, http://sp2007/sites/test/Shared%20Documents.
8.Under Location type, click the appropriate location type. In this example, select SharePoint Foundation.
9.Under Trust Children, select Children trusted if you want to trust child libraries or directories.
10.Click OK.

Microsoft has published a very good article on this which will be very usefull to configure remaining parameters regarding the same:
http://technet.microsoft.com/en-us/library/ff191194.aspx

1. The above mentioned link covers default parameters for trusted file locations
2. How to edit a trusted file location
3. How to delete a trusted file location

You can also trust a location by using Windows PowerShell:
http://technet.microsoft.com/en-us/library/ee428287.aspx

References:
http://blogs.technet.com/b/wbaer/archive/2007/02/14/configuring-trusted-file-locations-for-excel-services.aspx

http://www.stickyglue.co.za/index.php/archives/83

http://blogs.msdn.com/b/team_foundation/archive/2010/03/06/configuring-sharepoint-server-2010-beta-for-dashboard-compatibility-with-tfs-2010-beta2-rc.aspx