The problem here may well be that you're running on an unsupported Linux platform. We don't test Scientific Linux so I have no idea if our licensing or application software will run on it. There's little I can do to help if it doesn't work.
Nevertheless, if your MGLS_HOME is set to .../modeltech/linux/mgls, it should be working. The messages mgls_admin is displaying are exactly what you get when MGLS_HOME is not set properly. Fortunately, you don't need to set MGLS_HOME to run Modelsim since Modelsim sets it internally. You should double-check the MGLS_HOME or try running Modelsim without it.