Do you have a SOLIDWORKS model with your simulation setup already applied to it? You may want to attach it to your post so we can take a look through it.
Make the LED with two extrusions 0.3 mm thick - un merge the extrusions (total is 0.6 mm). The surface between the extrusions is the junction.
The resistance from junction to the board side surface is 6 °C/W. Calculate the thermal conductivity of the material in the bottom extrusion so the resistance is 6 °C/W. Assign this material to the extrusion (the board side). Possibly make the top of the LED have a thermal conductivity of 10.
Activate radiation in the general configuration.
Assign the top of the led an emissitivity of 0.8
Some of the LED power will leave as light so subtract this from the power assigned to the led (about 6%).
Assign the LED power to the interface between the extrusions.
Assign surface goal on the interface between the extrusions (this result shall be less than 125°C).
** Ask the LED manufacture for a thermal design guide or if this process is correct.