...Run the game with the path to assets directory as the command line argument.
The assets directory contains the following sub-directories:
'monsters' - monster shapes '.mki3d' files
'stages' - designs of stages '.mki3d' files
'tokens' - token shapes '.mki3d' files
Just place the files with your designs in the respective sub-directories of the main assets directory. Shapes are selected randomly from each sub-directory for each stage.
The AppImage file 'mki3dgame-x86_64.AppImage' needs to be set executable on Linux systems. ( See instructions on http://appimage.org/ how to download and execute such files.)