You will get that error if you launch Maelstrom.exe and it does not know where the start.ini or Program folder for a mod. You must either copy the engine files to the file directory you placed the GOF 2.5 files (not recommended but it works), or launch Maelstrom with a shortcut, where the 'Start In' property of the shortcut is the path to the GOF 2.5 directory (this is the recommended way) that contains the start.ini and Program/RESOURCES folders.
