OpsManager local mode is documented here. It states:

You specify the directory in the conf-mms.properties configuration file and then install .tar archives of the binaries to the directory.

The '.tar archives' are actually .tgz files downloaded from the MongoDB download site. Using the name '.tar' is confusing, especially as it's in bold.

As for access permissions, the only requirements is that the files in this directory should also be readable by the mongodb-mms user - this is not clear from the docs either.