Does my Kimchi need to ferment longer? I made my first batch of Kimchi, it has fermented for about a week &amp; the bubbling seems to have slowed down. Can I eat it now or does it need to ferment longer in the refrigerator? 

Also, the liquid level seems to have dropped during fermentation such that the kimchi at the top of the jars is no longer under the liquid. Should I add more liquid or just leave it be? The kimchi tastes quite salty, if I add more liquid should I add salted or unsalted water?
You can eat kimchi at any point.  More fermentation/time simply means more funk.  It is more likely that things are floating, your only risk is mold forming on the surface.  You can add a weight of some sort to keep the exposed ingredients under the liquid, but if you are going to keep them sealed like that, mold formation is less likely, unless you remove kimchi with fingers or non-sanitary utensils.  You can also add brine, but I don't think that is as common in a kinchi situation, as it is in other pickling situations.