Down at the bottom right of SEP (well, that's where I've gto it in my window), you should see 'Loupe and Histogram'. Hover your mouse in that area and a wee sub menu will pop up saying 'Loupe' 'Histogram'. Don't worry about those. You're not using them just now. But below that you'll see a table that goes from 0 to 10. 0 is black and 10 is white. At the left of that there is a wee check box. Make sure that it ticked. Now ...................
Run your mouse icon thingy along that table/grid from 0 to 10 and look what's happening on the picture you have up in the main window. Come back and tell us what you're seeing?