The slingbox has a password. If you haven't set it when setting up the slingbox, it's probably still something like 0000.
Your user account on the sling website has a password. You chose it when you registered on the site.
These are two completely separate and different passwords. Make sure you're using the right one.
I have my browser set to remember my sling account website password - but if I didn't, it would ask for this one every time I tried to watch my sling box through the watch.slingbox.com website. So that's probably the password you need.
If the PC and Slingbox are on the same network when you first try to access it, the PC app should just find the slingbox automatically and sort everything out for you without any intervention. It'll set up the slingbox, router, and store the slingbox details in your sling account, all without you doing anything. Then next time it'll "just work" from anywhere in the world from your sling website account.
Hope this helps.
Cheers,
David.