You should get an email receipt and that I think has the account number also it is on that little card they send you in the welcome package they mail to you.
Though you don't really need it to setup your myexede account. You can go to my.Exede.net and click on new user and then click on the Mac address option. Now you can get that number in 3 places..bottom of your modem, off the box the modem came in(if the installer left it with you) or off the modem status page at 192.168.100.1 then you can use the email or phone number on the account to continue setup.
If that don't work send an email to firstname.lastname@example.org and they can help walk you through it and get you your account number. You should be able to continue though using the MAC address. Once logged in you can get your account number from there in case you ever need it.