You shouldn't need to share Sling accounts in order to watch. As long as you know the SlingID and the hardware view password for the Pro HD, that's enough.
If you need to find the SlingID for the Slingbox, have a look at this support doc:
To watch, you can use the full desktop SlingPlayer or the Web Player. You can download the full SlingPlayer here:
To use the Web player, go to watch.slingbox.com. You'll need to log in with your Sling account (email + password, same as your forum login).
Regardless of whether you choose to use the Desktop or Web Player, use the SlingID to add your Pro HD to the Directory and you should be all set.
Hope this helps,