How to add a checkbox form to a panel?
One way is to define your own form and add it to a block in the panel.

After that, you save the value to the user account in the form submit handler.

Background:

Form generation summary

Relevant hooks:


hook_block_info ( define form in a block )
hook_block_view ( return $block['content'] = drupal_get_form('my_module_user_notify_form'); )


and some custom functions you'll need.


my_module_user_notify_form ( return form array )
my_module_user_notify_form_submit ( save to user )


If you use something like Profile2, you could also just embed the relevant user profile form in the block. It would then handle the form build and processing for you.