How to Convert MPP to MPX
The MPP file format is a class of files created using any version of the Microsoft Project application. Different versions of Microsoft Project create slightly different MPP file types that can only be read by that version of Project or newer. In order to read an MPP file using an older version of Microsoft Project or using another project planning application you must convert the file to the MPX format, which is a generic, format-free version of an MPP file. If you don't have a recent copy of Project on your computer you can still convert MPP to MPX for free in just a few minutes by downloading a free demo version of the latest version from Microsoft.
Install Microsoft Project
Open your web browser and navigate to Microsoft's Project trial download website located athttp://www.microsoft.com/project/en-us/try-buy.aspx.
Click the "Try Project Professional" button.
Click the "Get Started Now" button.
Log in with your Windows Live ID, or click the "Sign up now" button to sign up for a Windows Live ID. If you use any Microsoft online service such as Hotmail, MSN Messenger or Microsoft Passport then you already have a Windows Live ID.
Click the "Download" button to save the Microsoft Project 2010 installation file to your computer.
Double-click the downloaded file to install the demo version of Microsoft Project 2010 to your computer.
Run the Microsoft Project Professional application.
Click "File" and then click "Open" to initiate the open file window.
Navigate to the folder that contains the MPP file, then click the file and click the "Open" button.
Click "File" and then click "Save As" to open a save window.
Click the drop down menu next to "Save as type" and choose "MPX" from the list. Click "Save" to save your MPP file as an MPX file.
Article Source: http://www.ehow.com/how_12196974_convert-mpp-mpx.html