I recently installed Matlab in
/usr/matlab. What do I do to make it appear in the application launcher on the top left of the taskbar?
You can add a launcher to the panel by right clicking on a free area on the panel and selecting "Add to panel" and then "Custom Application Launcher" (or if the application is already present in the applications menu, you can select "Application Launcher" and then select the application from the menu).
You can add an entry into the applications menu by right clicking on it and selecting "Edit menu".