When trying to apply Maintenance Release 7 for BlackBerry Enterprise Server version 4.1 SP6, you are prompted for the BlackBerry Entperise Server.msi file.
This is by design and is expected that the full BlackBerry Entperprise Server version 4.1 SP6 installation files are downloaded and unpacked in to their default location:
C:\Research In Motion\BlackBerry Enterprise Server 4.1.6

To resolve this issue, download the BlackBerry Enterprise SErver version 4.1 SP6 from Research in Motion, and extract to the default paths.

You may then proceed with the installation of the Maintenance Release,

This issue has been reported to Research in Motion, and is being reviewed by their support personnel.