Click the field to the right of the friendly name (Data Source table column), and then select a data element from the Suggestions tab (1). If you cannot find the data elements in the Suggestions tab, then click the All tab (2) and search for the data element you are looking for.
To the right of the data element name is the category where the data element is grouped in. You can use the search or filter field (3) to look for the data element.