I don't think that you need to be a Skywards member. I booked an EK flight for my sister on Y last week (she's not a member) and not only can I make online seat selection, I came back later to the booking and changed the seat selections again. Other than seat selection, you can also choose special meals as well ^
Go to "Manage a Booking" tab, key in your last name & booking reference. It should be able to see your booking. Maybe the problem is the website is still new and there are still some bugs that needs to be ironed out.