when password security-setting dialogue box is opened, the bottom can't be seen or moved

Using Acrobat XI Pro on my Dell PC, I was trying to add password protection to the document.  When I click on the "encrypt with password" button, the password security-setting dialogue box pops up, and I can enter the password.  However, the bottom of the page is not visible and I can't move it to click the "ok" button.  Any suggestions?