To enable MSI logging, launch command line (CMD) and proceed to where the backup agent MSI is located and run the msiexec commend.
Here's the parameters for msiexec for verbose logging
msiexec /i "C:\my stuff\package.msi" /l*v "d:\place for logs\package.log"
"C:\my stuff\package.msi" = Path to where the MSI is located
"d:\place for logs\package.log" = The location where the log will be placed
