it sounds like you have your sling account password correct. but the password for your slingbox either isn't stored in your sling account or it is stored in the sling account but it is wrong for some reason.
do you remember the admin password you used when setting up your slingbox? if not, you may need to reset the box and set it up again to get it working.
it is confusing. just remember that you have a sling account with a password. and then you have a slingbox that has a different password that was created during the slingbox setup process.
i hope this helps.