napari. A napari window should open.
.tifffiles, drag these into napari (onto the main window in the middle). This should be whatever you passed to cellfinder originally, i.e. a single multipage tiff, or a directory of 2D tiffs. You can load as many channels as you like (e.g. the signal and the background channel).
-oflag) into the napari window. The plugin will then load your detected cells (in yellow) and the rejected cell candidates (in blue). If you carried out registration, then these results will be overlaid (similarly to the brainreg plugin, but transformed to the coordinate space of your raw data).