I am pretty sure that this issue results from the fact that McNeel doesn’t create the User settings file that sets the Rhino IronPython path until the very first time that you open Rhino Inside Revit. So opening Rhino Inside Revit after you installed Ladybug Tools causes this import error.
Rerunning the whole installer after the first time you open Rhino Inside Revit should fix it (as @PaulWintour suggests). But a quicker fix is just to rerun the set_python_path.bat file inside your ladybug_tools installation folder.
