You can create OpenMI-compliant components using only the official OpenMI standard, which can be downloaded from http://www.openmi.org.
However, it is recommended that you take advantage of the OpenMI software development kit (SDK) provided by OATC. You can download the SDK from http://sourceforge.net/projects/openmi/. The SDK is programmed in C#, so if you are using the Java standard you cannot use the source directly, but you may still find it useful as a source of inspiration. The SDK also includes the OpenMI standard, which means that you do not need to download the openmi standard from http://www.openmi.org.
You may also find it useful to download the OpenMI configuration editor from http://sourceforge.net/projects/openmi/. Loading your OpenMI-compliant component into the configuration editor is an easy way to make the first test.