To view the information for a contact or a phone number, just move your mouse over the name or phone number of the message sender.
Contacts
For contacts, their details will appear.
Phone numbers
For phone numbers, you'll get their reverse lookup directory info. This works best for land line phone numbers; cell phone numbers will usually only show their city.
You can click on an address in their reverse lookup to view their location on a map.
You can click
to type in a name that will be displayed instead of the phone number. Note that this name is only on your computer and won't be synced back with your contacts.