You may get an error message like this when installing a new theme or module in Orchard CMS:

Package uploading and installation failed

There are a few things you need to check to find out what the problem is, make sure that:

• your ApplicationPool is using the .NET Framework Version 4.0 and an Integrated Managed Pipeline Mode.
• the ApplicationPool Identity is set to ApplicationPoolIdentity
• the user your web server process is running under has write access to the Modules, Themes and App_Data directories.

For the last point, you can find out the user by opening the task manager, showing processes for all users and checking the User Name for IIS Worker Process (e.g. w3wp.exe). Then give this user write access to the three folders mentioned above.