To run a test, you must first import or load a data file, then open or create a script that includes one or more Strategy sections.
If the currently active script includes a Strategy section, then the Test button on the Tool Bar (or the Test item on the Run Menu, to which it is a shortcut) will be enabled. Pressing it will run the script in backtest mode.
A Test Settings section can be added to a script used to ensure that specific settings are in place before a test is run.
If you haven't already done so, please go through Tutorial 1 and perhaps the others to get a feel for how backtesting works in RealTest.
For a look "under the hood", see Backtest Engine Details.