If ALL prints are too dark make sure you are in a well-lit area. The best test is to take it outdoors and view it in daylight.   If you view it in daylight and its still dark, you may need to calibrate your monitor at a lower luminosity.   120 CD squared is recommended for LCD but 80-90 may be better with newer LED Technology.  If your monitor projects too much light it may be setting false expectations on your print side.   If you want a quick fix on the printer side, Canon has a global adjustment setting for brightness in the Canon Export Plug in on the color settings tab:

IF you want to see what setting of brightness is appropriate choose the adjustment pattern setting button and print out a brightness adjustment pattern.  You can set the increments to show you incements of plus or minus and then set your brightness slider to make that adjustment to everything you bring into the plug in to print.  (Picture Below)

If you want to see what setting of brightness is appropriate choose the adjustment pattern setting button and print out a brightness adjustment pattern.  You can set the increments to show you a plus or minus and then set your brightness slider to make that adjustment to everything you bring into the plug in to print.