Assuming the XML file is already on your development computer:
If you have a file, file.xml, you can put the file in your top-level development directory (the one containing the 'source' directory and 'manifest' file, etc.) and it will be available from your code after you side-load it to the Roku. You can access it using the file-name: pkg:/file.xml
Or, you can create a directory under your top-level directory, e.g. 'xml' and put your xml files there. You can access them as: pkg:/xml/file.xml