You can't alter how SMS messages are displayed as the "chat-style" interface is integrated in to the operating system. The old style (separate messages by date) was lost when your phone software was updated from v4 to v5
Many BlackBerry owners said they didn't like the new chat-style interface. The only way to get the old style back is to wipe the existing device software off your phone and re-install the old v4, the problem with that being you'll find it difficult getting hold of v4 now.
Also, you might have some third-party apps that won't work on v4
I have blackberry curve 8520 when someone me send sms it shows as a chatting message, it doesn't separate SMSs on date bases.
Right now I have Option > SMS > Conversation Style > Bubbles
I need to separate SMSs for one sender on date bases currently they combine under one chatting session