A. ConfigMgr.manifest.cab contains details of updates available for the SCCM installation. It's not available as a standard download from Microsoft. Instead, you need to run a specific command on a machine that has Internet connectivity (it doesn't have to be a server—I used a Windows 7 desktop). The command downloads all the updates and can place them in a folder, which you can then direct the setup process to use for updates during the SCCM installation.
Navigate to SMSSETUP\BIN\I386 folder on your SCCM media from a command line and run the command
setup.exe /download <location>
where <location> is where you want to store the SCCM updates, such as c:\temp\sccmupdate. Make sure the download folder exists before running the command. Now copy this content to the SCCM server. When you're prompted for updates during the installation, select the option to point SCCM setup to an existing download of the updates. Your installation will continue.