OK, let me be more clear.
The wrestling ring file you downloaded is not a scene, and cannot be opened as a scene. It's an asset that you can add to a scene by loading it into a "customUnityAsset" (CUA for short) atom.
You have already put the assetBundle file in custom/assets.
Now load the scene you want the wrestling ring to appear in. Or create a new scene.
Now click the toolbar button that looks like a cube outline (top row, third from the right.)
This opens up add atom>misc>custom unity asset.
Check the 'select atom on add" box so that once added, the CUA menu will appear. Now click "add atom.
This adds the CUA, and opens its menu.
Click "select file" which opens up a file browser in custom/assets. Here you'll find the assetBundle file you saved earlier. Load that file.
Once it's finished loading (it may take a few seconds), you need to select which asset from the file you want to use. Down below, where it says "asset" there's a button that says "none". Click that to bring up a dropdown menu of all the assets in the file. Pick one.
Once that has loaded, your wrestling ring will be visible in the scene.