My button box only shows a rev bar, or a few LEDs in one corner, after installing the Button Box Profile

That is the Button Box Profile before the plugin has built it for your box. As downloaded, the file is a plain rev bar; the plugin turns it into your box's lighting on your PC, and until it has done that the box shows a short rev bar in one corner and nothing else.

It builds it the moment it holds your box's data, which it fetches from your account when it first sees the profile on a device and again every time SimHub starts. So, in order:

1. Sign in. The plugin's Home page must show you signed in. A signed-out plugin holds no box data at all.
2. Be online for that start. The data comes from your account over the internet. Once it has arrived it is kept, so later starts work offline.
3. Check your membership covers the box. Button box profiles are included from Pit Crew up, or with a partner code for that box. On a Free account the profile stays a rev bar.
4. Restart SimHub. Close it fully and open it again. The plugin asks for the data on start and builds the profile on its next pass, normally within the first minute. Nothing needs reinstalling or re-selecting.
5. Make sure it is on the right device. The profile builds for the device it sits on. On the box's own device in SimHub it finds its box. On an Arduino build such as a Box One X1 or SimDash SD8-BB1 the box must be plugged in when SimHub starts, because the plugin recognises those from the hardware.

If it is still a rev bar after a restart with all of that in place, open Diagnostics on the plugin's Home page and send the support bundle with your ticket: the log says exactly why the box was not built.