Sscanf won't process the data until the terminator character (which you can specify) arrives on the line. Either make sure your device is sending the expected terminator character as necessary, or change the terminator character to something else. That's the easy first thing to check.
Failing that, your next step is to verify that the data is actually being sent to the computer.
If you can, grab a scope and check your RS232 lines for activity. If you see none, check your hardware/wiring/firmware.
Then, grab a terminal emulator like
Termite and use it to view the incoming data. If you see none, you'll have some debugging to do (firmware bug, faulty wiring, incorrect baud rate, timing, etc.). If you see data, verify that it is what you expect.
If you see the data in your terminal emulator and it looks like what you are expecting, then the problem lies in your Matlab receiving code. I'm not familiar with Matlab. If you are sure the data is arriving and is in the expected format, check the Matlab documentation, make sure you are using the serial API correctly, see if you can find some samples that work.
Hope that helps,
Jason