You can't send an email to the seller, you can only send a private message via the ebay system.
From the listing page, you should see the "Contact Seller" link. When you click that, you should see a box like the screenshot below:

When you click the Contact Seller button from that page, you should see the message page, as per the screenshot below, where you can type your message:

Is this not what you're seeing? If not, can you post screenshots of what you see?