Multiple Selection Checkbox 
To add a Multiple-Selection Checkbox field to the User Profile form, click on the following to your left: User Profile Fields> Add New User Profile Field then select Multiple-Selection Checkbox from the dropdown menu.

The Multiple-Selection Checkbox form allows you to add a custom multiple-selection checkbox field to the User Profile form.
You must click Save for changes to take effect.
Add New User Profile FieldDisplay Page
User Contributed Notes: Multiple Selection Checkbox