It is a well-known issue with Gnome >=3.10. Unfortunately there is no fix or workaround for it yet.
Source:EDIT http://www.mathworks.in/matlabcentral/answers/125436-why-will-warning-error-messages-not-show-up-in-a-pop-up-window-when-i-hover-over-the-text-in-the-mat(link is broken) Source: http://www.mathworks.in/matlabcentral/answers/125436-why-will-warning-error-messages-not-show-up-in-a-pop-up-window-when-i-hover-over-the-text-in-the-mat
The only solution suggested by the answers on the Mathworks forum is to switch off Gnome and turn on some other desktop environment (Xfce, for example.)