You should get a copy of the A5E extension when you downloaded the Monstrous Menagerie. You will need to enable that extension when you start your campaign (right hand panel bottom right of the screen where you load or create a campaign). Once the campaign loads enable the monstrous menagerie in the usual way.
There are some differences in the A5E monsters which requires the extension to be loaded.
See the videos linked in the first post for more details.

