Follow the link in the event website or email to make your submission, see Making a submission.
If symposia are available for you to attach your submission to, you will see a list of available symposia after submitting your abstract and in your personal dashboard. You will see public symposia (available to everyone) or private symposia (you have been invited to submit to).
Attaching your submission to a symposium
When you have created your new submission and clicked 'Submit', the next screen will show a list of available symposia. Click ATTACH next to the symposium you wish to attach your submission to.
You can then click on the title to attach or withdraw a submission.