The Dialogue System is compatible with Unity 5.x and Unity 4.6.5+. The Unity Asset Store will deliver the appropriate version of the Dialogue System for your version of Unity. If you upgrade your project from Unity 4.x to Unity 5.x, please see the Upgrade Notes.
The Dialogue System contains precompiled DLLs that significantly speed up compile times. For Unity 4.x, they're compiled in Unity 4.6.5 and compatible with all versions of Unity 4.6.x. For Unity 5.x, they're compiled in Unity 5.0.1 and compatible with all versions of Unity 5.x.
You can also unpack the source code as described in How to Unpack the Source Code.
These are upgrade notes from Unity 4.x to Unity 5.x:
PixelCrushers.DialogueSystem.SequencerCommands
PixelCrushers.DialogueSystem
namespace
specifier around the class).