I need to use the ACX Check Plug-in to check my levels.

No. You can do it the older, longer, painful way. ACX-Check is a shortcut.

It should look something like this.
Screen Shot 2017-12-07 at 18.07.23.png
Yes. it’s text. It’s XML programming language.

;nyquist plug-in
;version 3
;type analyze
;name "ACX Check"
;action "Compute statistics about clip and compare to ACX requirements"
;author "Will McCown"

; 2015-07
; release 1.0

; based on code from the plugins:
; measurement.ny from
; 2010-08
; and in stats.ny from
; Steve Daulton.

You should drag it into plugs-ins. You may need to restart Audacity. It opens under Analyze, not Effects.

Are you following the AudioBook Mastering Suite?

There is a correction. You no longer select a timeline by clicking just above MUTE. You click in the blank box lower-right in the INFO panel.

That is by far the easiest way to prepare your work for ACX submission.