Custom Mode activated in Preferences, but no MySql or PHP options when creating new site

I have a client whose host is running older versions of both MySql and PHP. I’ve set up on site in preferred mode, but am now trying to set up a new version of the site using custom mode that matches their host’s specs. In Preferences, I’ve changed the new site defaults to the Custom environment, but when I add a site, it doesn’t give me the custom PHP or MySql options. What am I missing?