Guessing from the title of your question, I think you want to load a MAT file into the MATLAB workspace and convert it to a CSV file, which you want to use it for further processing.
If this is the case, you can follow this workflow. Let's say we have a MAT file 'abc.mat' (attached), which contains two matrices and you want to load the two matrices into MATLAB and create a CSV from the concatenated matrix:
C = [A, B];
The created CSV file 'test.csv' is also attached. You can then use 'csvread' if you need to read from the created CSV file. The doc for csvread is here
Hope this helps