To contact Seller Central, you have to go through the Help function at the top right of the Seller Central portal. Sometimes it takes some playing around to get to where you can actually write a message.
When you click Help at the top right, scroll all the way down to the bottom. You should see a button that says "Need More Help?". If you click on Get Support underneath that, you can then click "Selling On Amazon" and then you can actually type out a message to seller central.
For additional help contacting Amazon seller support, please email ALGO Support (Team@ALGOonlineretail.com) for some advanced tips and troubleshooting.