If you don't know how to use the command line arguments you could just copy everything from the Rainlendar's program folder to the target folder and make it a portable version as explained in here: www.rainlendar.net/cms/index.php?option=com_easyfaq&Itemid=26#faq2
If you don't want to do that you can create a new shortcut from the Rainlendar.exe and add the -c argument to the target line in the properties dialog. E.g.
"C:\Program Files\Rainlendar2\Rainlendar2.exe" -c C:\Temp
You then need to use the new shortcut to start Rainlendar.