I install MT 4.2RC1 in local host when i sign in my blog with my default profile and checked "remember personal info" when i want to submit a comment, receive an error that Include this sentence: "Can't call method "value" on an undefined value at lib/MT/App/Comments.pm line 1127." what is mean..?!

Have you refreshed the templates?
Do you see any JavaScript errors on the comment page?