Acrobat X Forms - Unlocking Password Protected

Good afternoon,
I'm in need of some assistance. I have a form that instructors use to communicate grades and notes with students. Using version 9 we had no issues with password protecting this form, which prevented students from being able to make changes. The teacher entered the password and made updates, and when they were done the document was still password protected. The system was working great.
 
We recently upgraded to version X. I now know how to protect the document by going to Tools - Protection - Encrypt - and selecting encrypt with password.
 
This is great. Our problem is that we can not find the location to enter the password in to edit the form field data. Clicking ont he pad lock on the left doesn't allow us to enter the password. Going back to the protection area of tools also doesn't allow us to enter the password in.
 
My question is - where do I go to enter the password to make updates to my form? I do not want to remove encryption entirely. Once the fields are updated I want the form to still be password protected.
 
Thanks,
Sarah