To use the source data, you also need to add the REST Request test step to the test case.
To put the source data to a request, you can use a property expansion.
Select the REST Request test step in the project tree and open the Form tab in the input section.
Right-click in the Value field that corresponds to the parameter you need to specify and select Get Data from the context menu.
In the Get Data dialog, select the Data Source test step and the property you need.
The property expansion will appear as the parameter's value.
Now you can add a data source loop.