Hi Emanuel - for students who've been using Kwiziq for a while, the easiest way to see this is to look at your Brainmap. New lessons will appear grey as there's no Kwiziq score yet. For students who have completed most or all of a level, any new lessons will start to appear in your Studyplan automatically because you won't yet have a score for them.
For new students, the lessons are harder to search for, but it's not really relevant for them to look. Lessons are introduced in a designed order for that level and if the student has yet to get a score for earlier lessons, they'll get to it when it's appropriate for them to. If they have already past that position, the new lesson will be introduced immediately.
Hope that helps!