Make sure the ashcroft jar is on your project's classpath: Ctrl-Alt-S -> Paths -> Libraries (Classpath).
Select Edit Configurations from the run drop-down box to bring up the Run/Debug Configurations dialog box.
Switch to the JUnit tab if it is not showing already. To use Ashcroft with one specific unit test entry, add the following string to the VM Parameters text box as shown below:
Better yet, go ahead and make Ashcroft a default VM setting for your unit tests. Click the Edit Defaults button.
Now Ashcroft will automatically police your unit tests and make sure that they don't break any of the Ashcroft Commandments.