Thanks, gilby.

Checked the debug and found the problem here:
[19-Feb-2012 12:01:15] PHP Warning: require(../includes/languages/english/extra_definitions/multi_xsell_box_defines.php) [<a href='function.require'>function.require</a>]: failed to open stream: No such file or directory in /home/content/52/6977852/html/admin8dr7kj34/multi_xsell.php on line 222
[19-Feb-2012 12:01:15] PHP Fatal error: require() [<a href='function.require'>function.require</a>]: Failed opening required '../includes/languages/english/extra_definitions/multi_xsell_box_defines.php' (include_path='.:/usr/local/php5/lib/php') in /home/content/52/6977852/html/admin8dr7kj34/multi_xsell.php on line 222
It looks like the "multi_xsell_box_defines.php" in the install package should be located at languages/english/extra_definitions/ instead of going all the way to languages/english/extra_definitions/YOURTHEME/ .

Works like a charm now. Thanks for this great addon!