Import Errors
In most cases you will not receive any import errors after importing Behavior Designer Pro. However, there are two common situations where you may receive an import error.
Graph Designer Definition
Behavior Designer Pro will not compile correctly if you import or switch platforms while the project already has compiler errors. A common situation is that one of the Behavior Designer Pro add-ons is imported before Behavior Designer PRo itself. In this situation you’ll receive compiler errors similar to the output below even though Behavior Designer Pro is in your project.
Assets\Opsive\BehaviorDesigner\Add-Ons\Shared\Demo\Scripts\ScenarioSelector.cs(91,16): error CS0246: The type or namespace name 'BehaviorTree' could not be found (are you missing a using directive or an assembly reference?)
The reason for this is because the GRAPH_DESIGNER scripting symbol hasn’t been defined. This symbol gets added automatically but it cannot if there are compiler errors within the project. To fix you can either fix the compiler errors (such as by removing the add-ons) or by adding the GRAPH_DESIGNER symbol to the Player Settings:

Unity 6.3 TypeTypeLoad Exception
If you upgraded your project to Unity 6.3 you may receive this exception after opening the Behavior Designer Pro editor:
TypeLoadException: Could not resolve type with token 010000d3 from typeref (expected class 'UnityEngine.EventModifiers' in assembly 'UnityEngine.IMGUIModule, Version=0.0.0.0, Culture=neutral, PublicKeyToken=null') Opsive.GraphDesigner.Editor.Views.GraphView.OnGeometryChanged (UnityEngine.UIElements.GeometryChangedEvent c) (at <bd9cf0b0ba5a4f9a855802b1ee8dd9d0>:0) UnityEngine.UIElements.EventCallbackFunctor`1[TEventType].Invoke (UnityEngine.UIElements.EventBase evt) (at <0bcb90c459a2413a94634a44aeb1d754>:0) UnityEngine.UIElements.EventCallbackRegistry+DynamicCallbackList.Invoke (UnityEngine.UIElements.EventBase evt, UnityEngine.UIElements.BaseVisualElementPanel panel, UnityEngine.UIElements.VisualElement target) (at <0bcb90c459a2413a94634a44aeb1d754>:0) UnityEngine.UIElements.EventDispatchUtilities.HandleEvent_BubbleUpCallbacks (UnityEngine.UIElements.EventBase evt, UnityEngine.UIElements.BaseVisualElementPanel panel, UnityEngine.UIElements.VisualElement element) (at <0bcb90c459a2413a94634a44aeb1d754>:0) UnityEngine.UIElements.EventDispatchUtilities.HandleEventAtTargetAndDefaultPhase (UnityEngine.UIElements.EventBase evt, UnityEngine.UIElements.BaseVisualElementPanel panel, UnityEngine.UIElements.VisualElement target) (at <0bcb90c459a2413a94634a44aeb1d754>:0) UnityEngine.UIElements.EventDispatchUtilities.SendEventDirectlyToTarget (UnityEngine.UIElements.EventBase evt, UnityEngine.UIElements.BaseVisualElementPanel panel, UnityEngine.UIElements.VisualElement target) (at <0bcb90c459a2413a94634a44aeb1d754>:0) UnityEngine.UIElements.VisualTreeLayoutUpdater.DispatchChangeEvents (System.Collections.Generic.List`1[T] changeEvents, System.Int32 currentLayoutPass) (at <0bcb90c459a2413a94634a44aeb1d754>:0) UnityEngine.UIElements.VisualTreeLayoutUpdater.Update () (at <0bcb90c459a2413a94634a44aeb1d754>:0) UnityEngine.UIElements.VisualTreeUpdater.UpdateVisualTreePhase (UnityEngine.UIElements.VisualTreeUpdatePhase phase) (at <0bcb90c459a2413a94634a44aeb1d754>:0) UnityEngine.UIElements.Panel.UpdateForRepaint () (at <0bcb90c459a2413a94634a44aeb1d754>:0) UnityEditor.UIElements.EditorPanel.UpdateForRepaint () (at <9a6866749bf64829b050cb97017b2ca9>:0) UnityEngine.UIElements.Panel.Repaint (UnityEngine.Event e) (at <0bcb90c459a2413a94634a44aeb1d754>:0) UnityEngine.UIElements.UIElementsUtility.DoDispatch (UnityEngine.UIElements.BaseVisualElementPanel panel) (at <0bcb90c459a2413a94634a44aeb1d754>:0) UnityEngine.UIElements.UIElementsUtility.UnityEngine.UIElements.IUIElementsUtility.ProcessEvent (System.Int32 instanceID, System.IntPtr nativeEventPtr, System.Boolean& eventHandled) (at <0bcb90c459a2413a94634a44aeb1d754>:0) UnityEngine.UIElements.UIEventRegistration.ProcessEvent (System.Int32 instanceID, System.IntPtr nativeEventPtr) (at <0bcb90c459a2413a94634a44aeb1d754>:0) UnityEngine.UIElements.UIEventRegistration+<>c.<.cctor>b__1_2 (System.Int32 i, System.IntPtr ptr) (at <0bcb90c459a2413a94634a44aeb1d754>:0) UnityEngine.GUIUtility.ProcessEvent (System.Int32 instanceID, System.IntPtr nativeEventPtr, System.Boolean& result) (at <b589a158f82a4d0383d140f349b608e5>:0)
This is due to the Unity 6.3 assembly changing. To fix download the latest version of Behavior Designer Pro from the Package Manager using Unity 6.3. There are different versions of Behavior Designer Pro submitted based on the Unity version.