Steps to enable Secure Store Service for Excel Using Unattended Accounts

A.
To configure you first Configure a data access account then Configure a secure store target
application for the unattended service account then Configure Excel Services Global Setting and then
Configure a workbook to use the unattended service account.

B.
In SQL server expand Security, right click logins, and then click new login. In the login name box
type the name of the active directory account. Next in Select a Page click user mapping, slec the map
then under Database role membership selck db_datareader. Click Ok to finish

C.
In Central Administrator select Application Management, click Manage service applications. On
Manage Services Application page, click Global settings. On the Excel services settings page in
External Data section, thpe the target Application ID in the Application ID box. Click Ok to finish

D.
In data-connected Excel workbook, on the data tab, click connections. Next, go to Workbook
connections dialog box, select the data connection that you want to update, and then click
properties. On the connections properties dialog box, on the definitions tab, click authentication
settings. On the Excel services Authentication settings dialog box, select the Non lption and then
click Ok to finish

Explanation:
Resources: http://technet.microsoft.com/en-us/library/hh525344.aspx (There is a video you can
watch that explains how to configure it)
Using the unattended service account involves configuring an Active Directory account and granting
it access to your data, storing the credentials for this account in Secure Store, and configuring Excel
Services to use the stored credentials when it needs to refresh the data in a workbook.