Event Core is a program that does absolutely nothing! It's strength lies in the fact that packed in the zip file is the OPL source code that illustrates how the menu (and the default menu entries) are called, how the 4 side menu buttons work, and how to react to system messages. Think of it as a 'skeleton' that you can hang your own code round. With half the work already done, your development should be a lot easier and quicker.
You will want to look at The OPL Wiki to get more help on OPL Programming.
For those interested in an ER5 skeleton, the majority of the OPL code can be used in ER5 (remove the code for the CBA and replace it with code for the ER5 Toolbar). Alternativly you can go to RMR Software and download RMREvent, which is an ER5 OPL Event Code framework.
Download Event Core v1.10 for 9210 Communicators.
Download Event Core v0.30 for Symbian OS Series 60.
Download Event Core v0.61 for Symbian OS UIQ.