The download from java.com which installs in /Library/Internet Plug-Ins is only the JRE, for development you probably want to download the JDK from http://www.oracle.com/technetwork/java/javase/downloads/index.html and install that instead. This will install the JDK at /Library/Java/JavaVirtualMachines/jdk1.7.0_<something>.jdk/Contents/Home which you can then add to Eclipse via Preferences -> Java -> Installed JREs.