This example line

<install_dir>/bin/credentialstool --username <username> --password

Should have sudo:

sudo <install_dir>/bin/credentialstool --username <username> --password

Using the bin/credentialstool requires access to the private "key" of the MMS installation. If you're using the packaged RPM or deb installation of MMS, then this private key file is located at /etc/mongodb-mms/gen.key and is filesystem-restricted to only user mongodb-mms. Therefore sudo/root access is required to use credentialstool.