Say you have the following slide structure:
Title -- Agenda -- Topic A -- Topic B -- Topic C
Say the topics consist of several slides each and you want to show the agenda after the last slide of each topic.
Frankly, the easiest way to do this is to simply copy and paste the agenda slide between the respective slides. As a visual indicator of where you are in the prezzo, you can then also highlight the current agenda item and give the audience an indication of how much longer they will be able to enjoy the presentation.
Alternative: You can define a custom slide show that features just the "Agenda" slide. (Slide Show > Custom Slide Show > Custom Shows > New > select Agenda slide).
Then, on the last slide of each topic, create a shape or link, which (either on click or on hover) shows the custom slideshow (Select text or shape > Insert > Action > Mouse Over tab > Hyperlink to > select custom slide show) ... (and then returns to the current slide).
You can copy the shape or link to all other slides where you want to quickly show the agenda. You can put an invisible shape into a corner, for example, and set it to display the agenda custom slide show on hover. Then click or hit enter to return to the calling slide and advance to the next topic.
A custom slide show can either branch to the end of the presentation or back to the calling slide, but it cannot branch to the next slide in the sequence after the calling slide. And this is where the custom show alternative falls short. You'll always need to go back to the calling slide in order to progress to the next slide in the prezzo.
The benefit of the custom show solution is that there is only one agenda slide, and if you need to change it, you only need to change it in one place and won't have to copy and paste it all over the deck again.
Frankly, I'd prefer the linear way of inserting multiple instances of the agenda slide, especially since you can give your audience an indicator of where in the agenda you are -- by highlighting the current agenda item, -- and since you can just click through the deck, without having to concentrate on clicking or hovering over special shapes or links.