Assigning Default Applications to a File Type on OSX

For Mac OSX 10.6:

1. Open a New Finder Window and browse to a file of the type you'd like to change the extension. 

2. Click on the file. Go to File in the top left and select "Get Info".

3. Go to "Open With" and expand that section.

4. Click on the drop down and select the program you'd like this file type to be associated to. 

5. Click on the "Change All" button and "Continue"

source: http://www.mathworks.com/support/solutions/en/data/1-18ELZ/index.html