Where is the 'X axis mirror' in edit mode?

Hey all, the problem I have is that I’m modifying a mesh and the changes I make to one side doesn’t apply to the other!

Now I can’t use the mirror modifier because it is already weight painted, and in 2.49, there was a button that would let you edit one side and have the changes mostly my carried over to the other. Where is it in 2.58?

Thanks in advance!

Press T and look in the toolshelf

Thanks! Many thanks I was looking under the properties panel