Click on the "search sequences" button. If you only have one .fit files sequence in the working directory, it is automatically selected. Else, the list will pop-up and you'll be able to select the sequence you want to use. When the sequence is loaded, two windows are opened: the grey frame control window, on the left of the image below, and the RGB composition rendering window, behind the main window on the image below. In the case of CFA files, no RGB windows will be open as the image is monochrome.