There are two ways to select a Parent Folder one is when you add a new folder then you will get an option to select Parent Folder on the uploading pop-up, you need to select the Parent Folder and your folder will be added, the second way is when you want to add a parent folder for an existing folder then you need to click right button on the folder click on Edit and in the pop-up you can select Parent Folder, you can select your parent folder easily.