If your BPMN file was created with yEd, you probably changed the import filter in yEd's file chooser dialog from "GraphML" or "All Graph Files" to "XML + XSL" by accident. In this case, simply change "Files of Type" back to "All Graph Files".
If your BPMN file was not created with yEd, you have to specify a XSLT stylesheet that transforms your BPMN file into GraphML. (yEd does not support BPMN files natively.)