Access from the application menu: File -> Import. Or press CTRL/CMD + I.
Here you have a bunch of different options to import different file types from various programs. Most of them will create a new show.
Clicking on "PowerPoint" and selecting a presentation will import just the text and create a new show, which is useful for importing song lyrics.
But if you want to import the presentation as it is, with all of it styles, you currently have to import the presentation as PDF. If you have PowerPoint installed you can export it as PDF inside the application, if not you can use an online "PPTX to PDF converter".
When importing a PDF file, all the pages will be converted to images, placed in the "documents/FreeShow/Import/" folder. And then added as backgrounds in a show.
A FreeShow project is a file containing a project and all of it shows, it's useful to easily transfer a project to another computer.
Import a calendar .ics file. It will be converted to the programs own calendar format, that can be accessed from the "Calendar" view. If you import the same calendar multiple times, only the new/changed events are imported.