Just thought I'd drop this quick note should you also find the settings on the Araxis website don't work with your .gitconfig, I went through many attempts but this simple version worked for me.

[merge]
    tool = araxis
[mergetool "araxis"]
    cmd = araxisgitmerge.exe "$REMOTE" "$BASE" "$PWD/LOCAL" "$PWD/MERGED"

… or alternatively …

git config --global merge.tool araxis
git config --global mergetool.araxis.cmd 'araxisgitmerge.exe "$REMOTE" "$BASE" "$PWD/LOCAL" "$PWD/MERGED"'

Note: I added the Araxis installation directory to my PATH environment variable so not necessary above.