Can't get it to work

I consistently get this error when I put either the weekly or monthly widget in place: Warning: call_user_func_array() [function.call-user-func-array]: First argument is expected to be a valid callback, ‘fitnessWeeklyStats::control’ was given in “/xxx/html/working/wp-admin/includes/widgets.php on line 196.” Someone else posted this problem in the support area but a solution wasn’t provided. Disappointing since it would be nice to have a fitness plugin that doesn’t require external websites.