I installed minibb using Softaculous, which seemed to work. However, when I try to use minibb there is no login field, only start, register .... The form fields that do show up seem to operate ok.

Try to add forums first. The login to the Admin panel is on a different page. If it's Softaculous, possibly it's not renamed and by default it's located under your root forum URL/bb_admin.php. The link to the admin panel also appears straight after you've installed the forums.

Enter this section, create at least one forum and then check the first page. As soon as you add forums, login fields should appear.