When you read the documentation for msgbox, it clearly states the the output is "Message dialog box handle returned as a scalar". So the output is a handle, which is useful for keeping track of graphics objects, but it is certainly not a string. Who told you that the output was a string?
Also note that simply calling msgbox does not make MATLAB wait for a response, but the code will continue running as soon as the box has been created. If you wish to for the code execution to wait for the user response, then you will need to use waitfor.
You could try something like this:
waitfor(msgbox(...))
But it really depends on what functionality you want to achieve. Can you please describe what you want to happen with this code, and how it should respond given different user interactions (e.g. they close the dialog box without clicking okay).
