If a Visual3D solution does not compile out of the box this can have many issues, but one seems to be very common:
Broken references

I will show you how to fix them now. I will take the Demo Assets and Standard Assets references as an example here, but this applies to all references.

If you got warnings like these in any of your projects you can be sure that the references are broken:

To fix these, delete the references Visual Studio warns you about, but keep in mind which ones you need to replace now.

Next follow the steps on the pictures.

Now you got two choices:

For all the engine references (the ones with Visual3D at the beginning) those are located in Projects/Visual3D folder. 

All the projects like Demo Assets and Standard Assets you need to change to another directory:

Now select all the references you need:

Now one last step for all the new references: