I started the installer by invoking my standard Java version by starting > java -jar jdev_suite_121200.jar
And there you go, just go through the installer step by step. Don't be afraid of creating a new oraInventory anywhere and at the question about the user permission, I just allowed everyone to make changes to my new installation since its only to play around.
After the installation I was very exited to get my hands on the new Version and started directly. Unfortunately I only got the message that we need Java 7 now. (Sorry for the German stuff in the pictures)
Looks like we need to get JDK 7 first. This should be downloaded here: https://jdk7.java.net/download.html
After installation we need to change jdev.conf, otherwise JDeveloper still won't start. It's located in your installation directory under /jdeveloper/jdev/bin/jdev.conf
Open it and find the line where JAVA_HOME is defined and change it to the following path:
Should look like this in the end:
Ok, now you can start your JDeveloper 12c experience ;)
Because I have to switch between different JDeveloper Versions, I make use of the -J-Dide.user.dir start parameter. On Mac I create a simple Alias for the JDeveloper.app thats located in the <install home>/jdeveloper directory. Just open it with a good text editor like TextWrangler and add the start parameter to point to your desired USER_DIR for this installation. You even can use different USER_DIR for the same installation this way.