I have the following in my .gitconfig file:
.gitconfig
[user] name = myname email = myname@gmail.com [core] autocrlf = true excludesfile = C:\\
Try adding the location of meld.exe to your PATH (e.g. C:\Program Files (x86)\Meld), and then in the Diff Command, just enter meld instead of the full path. See the gist How to use meld with Sourcetree on Windows.