Checking a checkbox using add_post_meta

Hi, I am importing data via XML into my WP website, populating Simple Fields with imported data. Can you tell me how I can check a checkbox using add_post_meta?

I can set the value to 1 using:
add_post_meta(1, '_simple_fields_fieldGroupID_1_fieldID_1_numInSet_0', '1');

But that does not check the checkbox.
I then also added:
add_post_meta(1, '_simple_fields_fieldGroupID_1_fieldID_added_numInSet_0', "1");

but still nothing. Any advice would be greatly appreciated!

http://ww.wp.xz.cn/extend/plugins/simple-fields/