.material files are supposed to be opened with Notepad or similar programs so it's all good. When you're typing in the mesh's texture/material it's not actually the name of the .material file but what's actually inside, to open it you use Notepad like you have already done.
The first line should look something like this:
material rockMat
The part in red is what you type in Object Maker. Make sure to include all capital letters, symbols and spaces.