hmmm APOTU is SteamVR-only, so it’s not like there’s modes to enable. No idea why steam wouldn’t pick it up. You could try running APOTU while SteamVR isn’t running, that should trigger it. Or (if that’s how you’re running it now) first startup SteamVR, then APOTU.
If you move the headset around by hand, does something show up on screen? Does the controller appear if you hold it up in front of the HMD? Been a while since I tested my PC with WMR, maybe update the SteamVR WinMR driver?
Sorry I’m not much help I’m afraid : (