Checkboxes don't typically have a name that begins txt. Are you sure it is a checkbox and not a textbox?
The type of the userform textbox and the variable must match with the code you are using. If one is a string and the other is a boolean then you will need to convert it.
For example, does ActiveDocument.Variables("plaintiff").Value return "True" or True? These are different - one is a string and one is a boolean. Can document variables hold anything other than strings?
Perhaps try to initialize the value with
Code:
Me.txtPlaintiff.Value = ActiveDocument.Variables("plaintiff").Value = "True"