The master is being used just to trigger builds which are then run on slaves.
The slave is configured as a dumb slave.

I add a Unity3D installation on the master named UNITY_WINDOWS (Manage Jenkins -> Configure System -> Unity3d) but get an error if I enter the windows path in the installation directory so leave blank.

I can then go to the slave configuration (Manage Jenkins -> Manage Nodes -> Select XXX -> Configure -> Tool Locations) and see (Unity3d) UNITY_WINDOWS in the drop down. I can enter the path here (C:\Program Files (x86)\Unity\Editor\Unity.exe) but this is not used when building.

In a free-style job I can also select UNITY_WINDOWS in the Unity3d installation name.

On running, the following error message is displayed:

FATAL: Executable not found UNITY_WINDOWS
Build step 'Invoke Unity3d Editor' marked build as failure