Do you mean the Mac Mail client that comes bundled with OS or OS X? Or do you mean the Comcast webmail interface? If you open Mac Mail it will appear in the Dock and you can leave it there if you like. If you mean the webmail, drag and drop the URL from the address bar of your browser down to the right end of the Dock. Understand that if you sign out of the account, all that icon will do is return you to the sign-in page.