Does any one know about the software to open MPP files?

MPP files are typically Micrsoft Project files. There is viewer available at Project Reader - Reviews and free Project Reader downloads at Download.com (http://www.download.com/Project-Reader/3000-2076_4-10747111.html?tag=lst-1) if all you want to do is view the file. Otherwise, you'll need Microsoft project.

How do open MPP files ?

If by MPP you mean the file has the extension ".mpp" such as "myprojectplan.mpp" then you need to use Microsoft's Project software to open and edit the file. Or, if you just want to view the file, you can use the software I mentioned in the link above.

There are alternatives to MS Project for working with MPP files. Here are three open source (free) alternatives (http://www.osalt.com/project) that will open MPP files.

The primary commercial competitor I'm aware of is Fasttrack Schedule (http://www.aecsoft.com/products/fasttrack/). If you're a Mac OS X user, you might also consider the Mac-only app OmniPlan (http://www.omnigroup.com/applications/omniplan/). Both of these apps will work with MPP files.

There are some project viewer choices out there, such as:

LiveProject, which is free (http://www.kadonk.com/freeprojectviewer.aspx)
Steelray and Seavus has viewers too (not free though).

How to Open .MPP files or View .MPP Files for free (http://www.ceveni.com/2008/12/how-to-open-mpp-files-or-view-mpp-files.html)

There are some project viewer choices out there, such as:
Free Online Demo of the Viewer for Microsoft Project Files (http://www.projectviewercentral.com/projectviewer/demo.html)