Hi,
As the Instructions Carolyn has linked you to it is a question of repeating the Original Set up.
However this does leave the Mac Version showing both iPhone Numbers as "Receive At" and "Start Conversation From" options.
If you untick the Receive At item then the number will disappear from the "Start From" drop down.
To tidy the Received At list you need to remove the .plist involved.
In Any Finder window use th Go Menu whilst holding down the ALT key.
This make the Library items appear in the Menu list.
Select the Library (this is actually a hidden folder inside your Little House icon and this process reveals it).
Navigate to the Preferences Folder.
In here find the com.apple.imservice.imessages.plist item
Drag it to the Trash and Restart the App.
You may have to re-add the Apple ID and accept the pops ups about the iPhone Number and Apple ID in use on the iPhone.
If you happen to be on Mavericks rather than Mountain Lion then the .plist is the com.apple.ids.service.com.apple.madrid.plist one.
9:36 pm Monday; March 24, 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