If the Megascans view in the Content Library is empty for you even though you have assets downloaded from UE-Bridge, the most probable reason for this is that the Content Library couldn't locate where you downloaded your Bridge assets.
Here is a short video showing you how to locate the Bridge "UAsset" folder and add that path to the Content Library.
Sometimes it takes a minute for the newly downloaded assets from Bridge to appear.
Also worth noting is that your assets downloaded from standalone Bridge or from the Megascans website are not supposed to be able to be seen inside the Content Library. We currently only support the UE-Bridge assets.
P.S, make sure to use the paper clip icon on the right in the preferences window to assign the path, do not paste it directly into the path box. And also make sure to restart UE and download a couple more assets from UE Bridge if it ain't working directly after setting the path.