replacing "myfile.xls" with your filename, which can be absolute or relative. This connects to a file on your local machine. You then create your PMF/EMF as normal and use JDO/JPA as normal.
Relations : A spreadsheet cannot store related objects directly, since each object is a row of a particular worksheet. DataNucleus gets around this by storing the String-form of the identity of the related object in the relation cell.