Have you created the field(s) in the administration?
You can add both checkboxes and radio-buttons, see this documentation article for form fields: Form Fields - Marketo Docs - Product Documentation
Thank you for your response. Where abouts do I edit this in the administration?
Before you can add a field to a form, you need to create the field inside your database so that when someone submits the information, there is a place to store the data on the persons' contact record.
Navigate to Admin > Field Management, and click on "New Custom Field" towards the top of the screen. You will have to pick the field type which will determine what type of data can be stored in this field (numerical, string, boolean, etc.), and give the field a name. The system will automatically create an API name which I'd recommend just using what gets generated unless you have specific reasons not to. If you're not familiar with database field types and what to pick for that option, check out the documentation here to learn what each type means and what data can be stored within each type.
Once your field is created, go back to your form and you will be able to pick the new field (note, this might take a minute or two to show up, so wait a few minutes if you don't immediately see it after creating the field).
Hope this helps!