I have installed sketchup make 2016 on my sons laptop running windows 7 home premium 64bit. When using the rectangle tool to draw a square and entering exact dimensions ie 3000mm x 3000mm, then hit enter only one dimension remains at 3000mm the second dimension changes so that the shape is more of a rectangle than a square.
You don’t need to resort to the Line tool. The Rectangle tool will work just fine. Look at the syntax of the dimensions when you are dragging out the rectangle. Type the same thing,. i.e. 3000,3000 or 3000;3000 depending on what you use for a decimal separator.
You don’t have to enter anything after the decimal. If the dimension you’re entering is 3000, just type 3000.
What units are you using? Look in Window>Model Info>Units. If you want to work in millimeters, set the units to millimeters. If you don’t want to see dimensions any more precise than to the nearest millimeter, set the Precision accordingly.