From the main page click "Spend miles". Select operator Aegean, the dates, and select the route. Then click "request".
A little scary as you would expect it to be actually requested at this point. However it then comes up with the following screen:
http://upload.xandrios.net/145925623...d%20search.png
The different number of miles required are because of routing and Y/C redemption, though I am not sure if you can actually check for both on a given date. You will also need enough miles in your account in order to proceed from here.