If $dirFiles only contains one msi you can use Zero-Config to install it. Simply make sure that $AppName is left “as is” ($AppName = “”), PSADT will run it with the default parameters found in the configuration file.
It will be installed using the following if you have not changed it and is using Zero-Config:
<!-- Installation parameters used for non-silent MSI actions. -->
<!-- Installation parameters used for silent MSI actions. -->
Another thing to keep in mind is that when you specify -Parameters you replace the default parameters (which should already contain /QN or at least /QB), i usually specify -AddParameters which adds the specified parameters to the default parameters.
If you could supply us with a few lines from the logs we might be able to figure out why the installation fails but right now the only thing i can think of is that the file is not located in the folder where $dirFiles points.