http://www3.aeroplan.com/help/contact_us.do
1-800-361-5373
With hours 7am-midnight ET.

I tried the "hidden" manage my booking site, could not see the active icon to change seat.
Tried the regular number, no luck "higher than normal call volume message.."

So, ended up calling their Aeroplan number and the agent picked it up less than 1 min. Finally able to change our seats!!