Crumpet needs to find the Return To Wonderland installation folder on your computer so that it can verify game files and save CustomLevels.
If this is your first time using Crumpet, it will try to automatically detect where the folder is, and put the folder path into the input box.
If the automatically detected value looks good to you, you can just click continue.
If the text box is empty, or the automatically detected value doesn't work, you will need to manually enter the right value.
You need to use your computer's file manager to find where Return To Wonderland is installed. The folder you are looking for is the folder that contains the CustomLevels folder.
Once you've found it, click on a blank space within the file manager's location bar. The location bar is the horizontal bar near the top that shows you which folder you are in. If you did it right, the text inside the bar will move and become highlighted.
The final step is to copy and paste the value into the text box on this page. You can now continue and use Crumpet!