It is possible to clearify the term "Maven Home" in the maven settings panel by adding a label pointing out whether it ought to point to the binary file, the directory containing the binary file or the settings folder (with settings.xml and repository directory) and maybe let the label "Invalid Maven installation directory" tell what is wrong with the chosen directory.
I'll add that Netbeans just seems to be broken here. Using NB 8.1 on OX X 10.11.1, no matter what I choose for the Maven Home, it just says its an invalid install directory.
For the record, I used Homebrew to install Maven. After that install, the directory path that Netbeans wanted was: