Designer is integrated with Guvnor currently. This means that in standalone mode it is only able to view existing processes in your Guvnor packages but not save them. There is a number of people in the community asking for this feature to be added (check some of the latest posts on the forum) so it should be addressed in the future. Currently to save processes in your Guvnor package you would create a new BPMN2 process in Guvnor which will load it in Designer, edit/modify it there and then save it in Guvnor and build your package.
Hope this helps.