Select the step test step in which you need to use the data variable. The parameters available for the step will be displayed in the centre panel of the Test Designer tab.
Either:
Click on the Value field of a step parameter and begin typing the Name or Value of the data variable, then select it from the drop down list, or
From the list of variables in the Test data panel, drag and drop a variable onto a step parameter, using as the handle.
After assigning a data variable to a parameter, the value of that variable will be used by the test step when the test is executed.
Step parameters always use the current value of a variable.