To confirm that you are not just missing a setting, go to the Tools ... Developers Tool Kit ...

In the Top input box enter:
MIN_DISPLAY_XSELL

select NONE from the dropdown and click SEARCH ...

Does anything come up?

If, not, in the bottom input box enter:
MIN_DISPLAY_XSELL

select Catalog ... and click SEARCH ...

do you see any define for MIN_DISPLAY_XSELL in the list?