Hi,
Apple has some apps that are what is called Sandboxed.
Sandboxing is a way of protecting the whole System from something happening to one app.
This means as well as storing the .plists for various apps in the ~/Library/Preferences folder it also has a Read and Write route to the same place.
In the Finder open the Little House iCon folder (your Home Folder)
Use CMD+ J keys together.
In the panel of View Options select the bottom item to reveal the Library Folder (this will make it show all the time)
Navigate this time to ~/Library/Containers/com.apple.iChat/Data/Library/Preferences
In here you will find the same .plists but you will notice they are Alias versions (they are Aliases to the ones in ~/Library/Preferences)
Now use the View Menu > Show Path Bar and the details of the folders you have gone through will be sown in iChat and text form across the bottom of the window.
Right Click the Preferences one and then select Get Info.
Check you Own this folder and have Permissions th Read and Write to this as well.
Moving back along the line of icons repeat the above for the Library, Data, com.apple.iChat and Containers folders.

8:40 pm Saturday; January 24, 2015
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