I'm not really sure, because your question does not give much specific information about your situation (such as the actual name of the driver, that would be nice to have). My best guess: you are trying to access some kind of driver file using this thing you provided:
And the following is the case:
It could be that in your 'filename' you have for example the full path of your file as follows:
If your file is not a file with the extention .mdd, but for example a .txt file, this could give erros if you do not specify the file extention in the filed 'type'. Another possibility is that the name of your file 'yourfilename' accidentally contains a dot (i.e.: 'your.filename.mdd'). That might cause MATLAB to read it as a struct (while it is not) and therefore throws you an error.