It looks to me like the path to the AllUsers.msi might be the problem.
Was Explorer downloaded directly onto your computer? If so, try providing the complete path to the local copy of the .MSI. It is downloaded to the user's %temp% directory by default. For example, on my computer, this command works for me:
"msiexec /I C:\Users\<user name>\AppData\Local\Temp\ArcGISExplorer\2500\Setup\AllUsers.msi INSTALLDIR=C:\Esri /qb"
Please let me know if you continue to have any issues with this.
Regards,
Lindsay