If you are using the Open RSVP setting guests won't be asked for dietary requirements/menu choices at the RSVP stage (unless their email address is already stored on your guestlist) as the system can't identify them so isn't aware of which events each guest is invited to.
When a guest completes the RSVP form, You will then receive a notification and be asked to accept the guests response and update the guestlist accordingly.
Once guests have been added to the guestlist with their email address they will be sent an email by the system with a direct link to login to the guest area. Once logged in, guests will be able to add dietary requirements/menu choices to their details.
We would recommend perhaps adding a note to the RSVP section of your website advising guests to click the link when they receive the email to update their requirements.
If you want guests to be automatically asked to make menu choices/add dietary requirements when they RSVP you will need to add them to the guestlist first (including their email address). This way the system will be aware of the events you have assigned them to and ask those invited to the event that requires dietary requirements/menu choices is attached to for their choices.