Configuring excel services for a document library can sometimes be a difficult task. IMHO, Microsoft has made the terms rather confusing which has made following directions relatively difficult. Once you can get a hang of those terms, I think it is relatively straighforward on what needs to be done in order to configure excel services on document library. Needless to say that excel services only works for excel document (thought, I should make this clear). Here are few simple steps that you can take to enable and use excel services.
Step 1: Enable Excel Services
Under Central Administration-> Application Management -> Under Office SharePoint Server Shared Services click on Create or configure this farm's shared services. If you have not created an SSP yet, this is the time to do so. Click on New SSP link on the toolbar. You would need to designate a name of this new SSP, a web application that this will be running for (tip: This needs to have an application pool that does not run under Network Service account), the database server that this SSP is going be running under and authentication or service account for this SSP. Once entered, click on OK and this should create a new SSP. Once created, click on SharedServices1 (SSP that you just created), and locate Excel Services Settings -> Trusted file locations.
In order to enable excel services for the document library, you will need to add document library list to the list of trusted file connections. e.g (http://Contoso/Marketing/Expenses). While creating this, make sure that you choose "trusted data connection libraries only". This will restrict the excel workbooks to query data against trusted connection that you will create in Step 2.
Step 2: Create Trusted Data Connection Library
On Marketing site, on Site Actions menu, click on Create. Under Libraries, you will see a link to Data Connection Library. Create a new data connection and name it as MarketingDCL by ensuring that the document template as Microsoft Office Excel spreadsheet.
Step 3: Add Trusted Data Connection Library to Excel Services SSP page.
Click on Trusted data connection libraries and add a new Trusted Data connection library. On Address field enter /Contoso/Marketing/MarketingDCL (tip: no http).
Step 4: Create a new data connection file.
For this particular step, you will need to open Excel that already has an external data connected to it. For this exampe we will assume that excel is connected to Northwind database. In this step, your focus is to create data connection file and make SharePoint aware of it. In order to do that, In the Excel workbook click on Data tab -> Connections. This should pop up the window(Screenshot below). Click on Properties -> Definition tab click on Export Connection File. Type the path to the new data connection library that we created in step 3 (http://Contoso/Marketing/MarketingDCL/Northwind.odc) and click Save.
Now, ensure that the connection type is SharePoint Library and UDC purpose is ReadWrite.
Thats it. Now you can test your connection. Open a workbook and click on Edit in Excel. Once open, go to Data tab, click on Get external data Click on existing connections. In connection files on the network, you should only see the connections that are enabled on SharePoint library.
Thats it. This is all it takes to get the excel services configured and going on your SharePoint environment.