Usually I forget this step during code migration from environment to other. So I note it down for future reference.
The best way to move labels from environment to copy the *.lac file form location
C:\Program Files\Microsoft Dynamics AX\60\Server\MicrosoftDynamicsAX\bin\Application\Appl\Standard
And paste on some common folder in target environment.
And then import it by right click on aot => labels =right click and create from file.