Sorry for the confusion. Did you create your project new in UE 4.24? (if so, which category & template did you pick?) … or did you open a project that was made in an earlier engine version?
If the Datasmith importer is not automatically enabled, then for the plugins to enable, you will want to enable the “Datasmith Importer”, and potentially the others depending on the file formats you intend to import. (“Datasmith Content” is a default for reading files which have already been imported) Afterwards, you will see a little prompt at the bottom:
Choose “Restart Now”, and when you load the project on restart, you should see the button in the toolbar.