Is the box field in your internal table, if yes where is the problem, just set it to 'X', so I will suppose there is no such field and you are using OO and not REUSE FM.
Which OO tool are your using, look for a method like CL_GUI_ALV_GRID->SET_SELECTED_CELLS or CL_SALV_SELECTIONS->SET_SELECTED_ROWS.
Regards,
Raymond