The main Project Schedule app only supports a limited set of file formats. This addon adds support for MS Project files (mpp file extension) and MS Excel files. MS Project files can only be read. Writing changes back to a mpp file is not supported. To get your project back to MS Project you can export a MSPDI-XML file. Excel files can be read and written.
Opening mpp files of complex projects can lead to an out of memory error. In this case try to save your project as an xml file and import it with "MSPDI-XML-Import".
The addon is available from the following sites