I solved this problem using 2014b. 2015a doesn't have required files unfortunately. I posted this answer to a similar question on Matlab Answers, you may check there if this solution doesn't apply to you for further recommendations. 

(http://www.mathworks.com/matlabcentral/answers/183899-how-to-restore-file-association-in-matlab-2015a-for-windows)

So, I can recommend 2 methods:

1) Copying 3 fileassoc files from

C:\Program Files\MATLAB\R2014b\mcr\toolbox\matlab\winfun\private

to the corresponding R2015a folder and running

fileassoc('add',{'.m','.mat','.fig','.p','.mdl',['.' mexext]});

on 2015a in the corresponding folder worked for me. Then you are able to see R2015a in the Windows default programs list i.e.

Control Panel\All Control Panel Items\Default Programs\Set Default Programs

Matlab starter's icon disappeared though. I checked registry of Windows, and command just mirrored whatever is done for 2014b to 2015a.

2) If you don't have those 3 files, you may export registry settings for Matlab in 2014b, modify and run it if you installed 2014b on that machine. Backup whole registry before doing this. Registry settings are located at:

HKEY_LOCAL_MACHINE\SOFTWARE\MathWorks\R2015a

What is your OS? Have you tried "open with" and binding MATLAB to *.slx and *.m files?

I solved this problem using 2014b. 2015a doesn't have required files unfortunately. I posted this answer to a similar question on Matlab Answers, you may check there if this solution doesn't apply to you for further recommendations. 

(http://www.mathworks.com/matlabcentral/answers/183899-how-to-restore-file-association-in-matlab-2015a-for-windows)

So, I can recommend 2 methods:

1) Copying 3 fileassoc files from

C:\Program Files\MATLAB\R2014b\mcr\toolbox\matlab\winfun\private

to the corresponding R2015a folder and running

fileassoc('add',{'.m','.mat','.fig','.p','.mdl',['.' mexext]});

on 2015a in the corresponding folder worked for me. Then you are able to see R2015a in the Windows default programs list i.e.

Control Panel\All Control Panel Items\Default Programs\Set Default Programs

Matlab starter's icon disappeared though. I checked registry of Windows, and command just mirrored whatever is done for 2014b to 2015a.

2) If you don't have those 3 files, you may export registry settings for Matlab in 2014b, modify and run it if you installed 2014b on that machine. Backup whole registry before doing this. Registry settings are located at:

HKEY_LOCAL_MACHINE\SOFTWARE\MathWorks\R2015a

create a folder ...and include both files in it,,after that open m file first,, then .slx file so you can run it simultaneously,,,

Hi

If you have MathWorks MATLAB R2015a (x86 or x64) installed in your PC then you might be facing file association problem. All your previous MATLAB files won't be associated with new version of MATLAB and the supported file extensions won't be appearing in Default Programs option of Control Panel. Here I am providing you the solution for this problem.

1. First download these files:

For 32-bit Users [Download Link (x86)] (This archive contains fileassoc.m, fileassoc.mexw32, fileassoc_mexw32.auth)

For 64-bit Users [Download Link (x64)] (This archive contains fileassoc.m, fileassoc.mexw64, fileassoc_mexw64.auth)

Note: These files are created by MathWorks. I just copied them from the previous version (R2014b) and it is working with R2015a.

2. Then Paste these 3 files to this location of your PC.

3. Now, open MATLAB R2015a as an administrator and run the following code.

4. Tada! All MATLAB files are associated with the program.

Best regards

Hi Murat Ates,

I use MATLAB 2015a on Windows 8 x64 and, Method one has worked for me, thanks a lot!

Regards

hi Ahmed Guerine

 Its work very nicely...

Thank you all

It really works! Tnx a lot! (MatLab r2014b).

All that I need is to open the folder ("C:\Program Files\MATLAB\R2014b\mcr\toolbox\matlab\winfun\private") in MatLab "Current folder" and type in command line: " fileassoc('add',{'.m','.mat','.fig','.p','.mdl',['.' mexext]}); "

MatLab need to be run as an Administrator!