After downloading, double-click instantjchem-VERSION-macos.dmg to expand disk image.
In the extracted image, you will find the installer that you can launch by taking a double-click on it.
The Instant JChem application will be placed into the specified place. (Its default place is the /Applications directory.)
IJC requires 1.5 (or later) version of OS X Java. Java 1.5 is available since OS X 10.4. (Earlier OS X versions are not supported.) If the required Java is not installed, select it from the operating system’s Update Center to install.