The general approach to using MECSim is to download a variety of input files ("Master.inp") covering a range of electrochemical mechanisms and edit them for your particular problem. The best general help that is provided with the code is the user guide, which is found here.
An old presentation that explains one of the previous versions is available here. The approach is largely the same, although the input file format has been condenced and simplified, and MECSim has more capabilities.
A list of "Master.inp" templates for the MECSim package are given in the zip file here. In addition to the templates themselves there is also a PDF file which explains the mechanism for each template.
If you have gone through the user guides and tried a few things out then you can always email me on: