The custom roles will show up when adding/editing authors both from your book-editing page and from the public/common work page.
To add to or remove the sample roles that come with the script, select the "Add custom roles..." option, which will pop up a prompt where you can change what's currently there. Likewise, to hide roles you never use from showing up in the list, click "Suppress standard roles...".* None are suppressed by default.
The roles you add or change will be saved in the browser (localStorage) so that they stick around in the future.
* For those using non-English LT: if you need to use a role that you've suppressed, it's better to unsuppress it instead of adding it again via "Other...". This is because of an odd bug in LT that forgets such roles on subsequent edits.