I opened the Naviextras Toolbox program and was prompted to download an update. After doing so, it finally recognized my USB stick to download updates for the 8100nex.

 

Also, the 8100nex is finally listed on https://pioneer.naviextras.com/so you can register prior to getting the update tool working.

 

Of course, I found this all out after having purchased an SD card reader earlier today...