In DragonVale for the iPhone and iPad, the Plasma Dragon is a hybrid dragon with the water and lightning attributes, similar to the Current Dragon.
To get a Plasma Dragon, you need to breed a Lightning Dragon with a Water hybrid. Some examples are Lightning + Fog, Lightning + Rust, etc.
Note that any of these combinations do not guarantee results, and the different combinations have different percentages for success. Keep trying though and you’ll eventually get your Plasma Dragon.
Check out my DragonVale Breeding Guide for more dragon breeding combinations as well as other Water hybrid combinations you can use to get your Plasma Dragon.