Select Shadow Mesh
As you can see, the sunshadow mesh isn't yet adjusted.
Change to the mesh tab where you've previously imported the high and the low detail mesh. From the dropdown menu select Shadow high level of detail.
Press Generate Shadow Mesh Button
Click it to open the Select MLOD window. You have to tell TSR Workshop what should be all included in this shadow mesh from the High level of detail mesh. What generally should be excluded is the groundshadow mesh (you can recognize it on the low vertices/faces count). When you're done selecting the meshes, click on OK.
The newly generated sunshadow should now become visible in the 3D Preview. If not, make sure you are in the correct LOD mode and have sunshadows showing activated . In the Mesh Tab you've also got a rotation slider available so you can check how the sunshadow looks from different angles.
Repeat for Low LOD
Now repeat step 1 and 2 for the Shadow low level of detail. This time you choose the meshes from the Low level of detail mesh in the Select MLOD dialog.