If your data set does not have turn restrictions, but you still want to show the one way street direction, then you can do this clicking on the Import Turn File button. When you are prompted for a turn.txt file as shown below:
Click on the "Cancel" button and you should see a prompt asking if you would like to import one way streets and any road restrictions that may have been defined on your network:
If you click the "Yes" button then your network will be scanned and any one way streets will be displayed.