Your settings in the diff configuration are almost correct, you just need to make a little adjustment to make sure that (for example) git can find the binary. In the git docs, it states that the binary (in your case mvimdiff) must be in your path or else given its full path.
That leaves you with two options, either make sure that mvimdiff exists in /usr/local/bin or /usr/bin (or some other place that is on the Mac's default path), or do what I have just done and tested: put the mvim script in a known place in your $HOME directory and reference it there with the ~ (the only expansion that git supports).
So, I have created a folder in my Home folder called .bin (with a leading dot to hide it from the Finder) and then put the mvim script in there. Then in SourceTree I have set the 'Diff Command' to
~/.bin/mvim -d
and I have set the 'Arguments' similar to what you have, but I have put them in quotes to handle files with spaces and other special characters in them, so
"$LOCAL" "$REMOTE"
NOTE: I have not tested this with hg or subversion, but since svn is supported via git-svn, it should work for subversion in that configuration too.