Jenkins doesn’t load under OS X Mountain Lion (or Server)? Here is the fix!
Answer: your machine doesn’t have Java installed on it. This happened to me. Jenkins installed fine, but then when I opened it on its default port of 8080, nothing happened.
To fix it, open Terminal and type “java” without the quotation marks.
This will tell you that you have to have Java SE installed on your computer and then will download and install Java SE automatically for you. Done! :-)