Linking a model from an external file with a rig

Hi guys,
I’m currently in the process of making a little animation, and am having a few problems bringing in assets from external files that contain rigs. Specifically, I found these two models on blendswap that I’d like to use:

This is the first time I’ve done this, so I may be missing something really simple, but I looked at some tutorials on YouTube, and the general consensus seems to be that you link the model, choosing the collections that contain the model itself, and the rig, then create a proxy between the model and the rig.

This is where things seem to start coming undone - when I try to create the proxy to the rig with either of these two models, the rig itself doesn’t seem to appear in the list of items I can make the proxy to. I’ve tried this in my working animation file, and also in a new, blank blender file, and I’m really struggling to see what I’m doing wrong! If anyone is able to shed some light on how I might go about importing these models along with their associated rig, it would be much appreciated!!

Many thanks in advance,
Rufus