Exception in thread "main" java.lang.UnsupportedClassVersionError (Trying run Alda on OS X)
There is a large highlighted first line of the installation instructions "You must have Java 7+ installed on your system in order to run Alda."

So install the JDK (not JRE) from Oracle as per Oracle's comment


  The Java Runtime (JRE) that you download from java.com or oracle.com contains a plugin to run Java content from your browser. In order to use the command line tools, you will need to download the Java Development Kit (JDK). The JRE and JDK are separate and can coexist on your system. Only one JRE can be installed on Mac OS X. There can be multiple JDKs installed on a system, as many as you wish.