Matlab *.mat files associated with MS Office
Solution 1:
Start->Default Programs->Associate a file type or protocol with a program.
Solution 2:
Right click an example file.
Choose 'Open With' on the menu then 'Choose Program'.
In the new window, choose MatLab & click the "Always use the selected program to open this kind of file" and click 'Open'.
The program should now be associated..
Solution 3:
In Windows 7, go to control panel, Default Programs, Associate a file type with a specific program
This should give you a list of file extensions and their associated applications. Select the .mat extension, then click change program (button at upper right) and browse to select the 'matlab' executable.
Here's the MS ref