By default, the mouse tool mode in L3DT is the information tool, which is represented by a standard arrow cursor. This mode may be activated by selecting the 'Tools→Default (info)' menu option, by clicking on the icon shown below, or by right-clicking the mouse in the main window a few times.
Double-clicking on a pixel in a map with this tool will open a small information box, showing the particulars of the chosen pixel. In the case of the design map, you may edit the pixel values directly.