How can I see the sky dimension?
It is possible to experiment with the sky dimension, but only through the use of mods. The minecraft wiki details how to go about this. Keep in mind, though, that this is experimental and should not be trusted or considered very well tested.
Notch's plans for the Sky dimension were put on halt during Beta 1.7.3. Since then, the world ID and code for the sky were taken by the End dimension. Even it there was a "sky portal" it would take you to the End. The Sky world does not exist and does not get created at the world start of SSP or SMP.