To select a Datasource, hover over the Datablock header and click the 
icon.
Use the "From Measure" to select the desired measures for your Datablock.
If you have an existing report that contains the measures desired, you may use the "Select Report" drop down menu select an existing report from your list.
Some Datablocks, such as the Drillthrough Datablock, require that you click the 
icon to access the Datasource.
