Sorry this may be too late.
I could not find a way to do this in Arc, so I programmed it in Python. If you are still working on this I can point you to some resources I used.
Hi Clay. I am having the exact same problem trying to use the Apparent Reflectance function for Landsat 8 imagery in Arc - nothing has worked and I am pretty much giving up and just want to write a Python script that does it to see albedo. Do you happen to have those resources you mentioned or would you be willing to show an example script?
I was using pre-LS8 data, so I'm not sure what is different between the resources I used and what you need.
The information you need to understanding converting: Landsat 7 Handbook
Table for if you wish to convert the thermal band(s): Landsat 7 Handbook
An example conversion script: skochaver/reflectance-tools · GitHub
An example if you're into GDAL (I used the metadata reader concept from here): Landsat DN to radiance script using GDAL and Numpy.