Hi,
Open any Finder window (Make the Finder the "front" app).
Use the Go Menu whilst holding down the ALT Key.
Select the Library that appears in the Menu.
Navigate to Preferences.
Find com.apple.ichat.plist (Messages still uses this)
Drag it to the Trash and restart Messages.
For many years this has given access back in iChat and Messages 7.x.x in Mountain Lion.
However it is not the instant "Cure" in Mavericks and Messages 8 that it used to be. (it may not work).

7:53 pm Sunday; March 16, 2014
iMac 2.5Ghz i5 2011 (Mavericks 10.9)
G4/1GhzDual MDD (Leopard 10.5.8)
MacBookPro 2Gb (Snow Leopard 10.6.8)
Mac OS X (10.6.8),
Couple of iPhones and an iPad