Hello @virginiasdolls and welcome to the forum 
You need to download ffmpeg and then tell krita where ffmpeg is so that krita can use it as a ‘helper’ utility to decode the .mp4 file.
To do that, you render an animation and tell krita where ffmpeg is in the Render Animation window.
After that, the location of ffmpeg will be remembered for other operations that need ffmpeg.
The plugin by Scott Petrovic (which also needed ffmpeg) has now been incorporated into the File → Import Video Animation facility.
You can get ffmpeg for macOS from here: static FFmpeg binaries for macOS 64-bit
I suggest that you get this one: https://evermeet.cx/ffmpeg/ffmpeg-6.0.7z
It will be downloaded as a compressed .7z archive file so you’ll need to Extract/decompress it to yield a folder containing the ffmpeg, ffprobe utilities.
I don’t know if macOS has a .7z decompression utility.
if not, another macOS user will be able to advise you about that.