AFNI operates on datasets which it finds in the current directory (if you don't type anything but "afni" on the command line) or on datasets it finds in the directories you type on the command line (as in "afni a b c", which will scan directories -- AKA "sessions" -- for 3D and 4D datasets). However, if there are
no datasets available, then it opens with a "dummy" dataset. In this case, the "Switch" buttons don't work, since there is nothing to switch to. However, you can view the dummy dataset -- try opening an Axial image window for example, then scrolling with the "Time index" control.
RW Cox