Install Meld from their webpage http://meldmerge.org/
I had to tell git where it was:
git config --global merge.tool meld
git config --global diff.tool meld
If using windows 8 and 10 use the following
git config --global mergetool.meld.path "/c/Program Files (x86)/meld/meld.exe"
git config --global difftool.meld.path "/c/Program Files (x86)/meld/meld.exe"