Have you read the help instructions (upper right corner)? There it is mentioned how and what to load for any given tab as they currently work independently.
Loading on the slicer tab is a little bit hidden as it is done via the the buttons that seem like the title (e.g. "Front", "Left", etc.), which opens the file explorer for loading individual images.
While, in the planner there is currently only the option to load in batches of the full view and not individual images (see instructions as well).
You can download the test_images in the download section to get something to start with as there are also examples for the different conventions.