Unlike Windows, X11 does not always automatically unify the keyboard. In other words, it does not map the "+" on the keypad to the same key as the "+" above the "=". Fortunately, the keyboard is completely modifiable under X11. You can set any key to pretend it is just like any other key using the command xmodmap.
The following is pretty advanced stuff. Be careful. Read the appropriate manual pages ("man xmodmap") and always work on copies.
If you run:
xmodmap -pke
It will send the current mapping to standard output. So if you run:
xmodmap -pke > MyKeyboard.txt
it will send the current mapping to a text file name "MyKeyboard.txt". You can then edit this file with your favorite text editor. For example, there is likely to be a line in the file:
keycode 86 = KP_Add XF86_Next_VMode
That is telling X11 that when keycode 86 comes in, it means "send" the keypad Add ("+") symbol to X11. If you change this to read:
keycode 86 = plus XF86_Next_VMode
Then if the keycode 86 comes in, it will send the regular plus sign "+" (the one above the equal sign) to X11.
Once you have made the appropriate changes to your text file, you can load up your new keyboard mapping file by using:
xmodmap MyKeyboard.txt
Of course, always work on a copy of the original file, so that if you make a mistake you can restore the original keyboard mapping from the original file.
That should hold you, while we get Sesame not to care which of the two "+" keys you pressed.