On the DirectAdmin homepage, you need to click on the "Password Protected Directories" option under Account Management;
Then under options, you need to click "Find A Directory to Password Protect";
You then need to find the folder "stats" and select the folder.
You will now be prompted to enter a username and password. Once you have done this click "Protect", and your stats folder will be password protected. The next time you try to access www.yourdomain/stats you will be prompted to enter these details.