Welcome to the Forum. Let’s see if we can get you some help…
Did you try importing using the File > Import > Bio-Formats command to open your files? Bio-formats allow importing data from life science file formats - including .avi files. It comes with the Fiji distribution of ImageJ.
NOTE: Fiji is Just ImageJ - it is simply a distribution of ImageJ that comes with a bunch of plugins bundled - ready for you to use out-of-the-box. If you are just getting started, we recommend downloading/using Fiji.
Let us know if this helps.