Change the 3 lines to:

if ($num_cross_sell == '1') define($mxs_selected_table = 'Cross Sell', '');
if ($num_cross_sell == '2') define($mxs_selected_table = 'Upsell', '');
if ($num_cross_sell == '3') define($mxs_selected_table = 'Related', '');

I've literally just fixed this problem and it worked for me.

Bob