You can click the browse buttons on the right hand side to bring up the Open File Dialog to browse for new files or you can select the "Add Files" item in the File Menu to add a batch of schemas. You can click the column headings to sort by that column.
The first column contains check boxes which can be used to temporarily disable a schema from being used in validation.
This list of schemas is persisted in the
XmlNotepad.settings file so it is remembered next time you load XML Notepad.