You will need to install Security Assist in order to select the administrator user.
"I do not have security assist installed, and the version that I do own is an old one."
Security assist version 1.1.9 is the version that should be used for this, if the "old one" is the version you have, that is the version available to you in the download center. you do not need to use version 2.
once you have installed security assist you will be able to select the administrator user and proceed
as for the error message, please send a copy of the CSSMenuWriter\cssmw\menu .php file so i can see the code at line 6