Can you download and install this version (direct download):
www.sketchbook.com/thankyou
NOTE: There are 2 versions of SketchBook on Mac.
a. Mac App Store: Applications > SketchBook
b. Direct Download: Applications > Autodesk > SketchBook
Then reset it to default.
SketchBook > Preferences > Factory Defaults
NOTE: Backup your brushes and colors from the color editor.
http://help.autodesk.com/view/SKETPRO/ENU/?guid=SKETPRO_Help_sb_brushes_use_brush_library_export_bru...
NOTE: Also use the default color editor before using the MAC color editor.
http://help.autodesk.com/view/SKETPRO/ENU/?guid=SKETPRO_Help_sb_colors_color_editor_html
David Lau