Pressing the button Import starts the actual importing process. To start the imported process at least one PDF file must have been selected. Depending on the size of the files and especially on the number and size of the images included in the PDF files this will take a few or up to many minutes.
You will see the status of the import process from the progress indicator visible while the importing process is performed.
You can abort the importing process by pressing the abort button on the progress indicator.