When you enter your personal data, before you click Continue, on the bottom of the page you have to check the tickbox for ”Doresc să îmi aleg serviciile speciale gratuite / I wish to choose my free of charge special services”. After you click Continue then you will have option to choose your seat and special meal options.
as other mentioned before, sometimes you can choose the seats on KLM/AF website, but it not working all the time. you can do it at OLCI, better choose on m.tarom.ro