You've got to be not clicking on the correct divider - the cursor will change to a left-right arrow & it's the one I've got the arrow pointing to which needs to be clicked & dragged to the right to expand the area. As I can't get any of my test setups to not do that.
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If you still can't get it to show (maybe there's a mouse / OS dpi setting that's breaking things), you'll need to open your gen_ml.ini when wacup isn't running & remove the whole plsplitdiv line from the file. Preferences -> Advanced -> Diagnostic -> Settings Locations tab will show you the part for your gen_ml.ini file.