You can find the user dir by clicking (1). There you can navigate to the data dir (2). There you should find a directory "proxymeshes" (3). If it doesn't exist, then create it. Simply place the relevant files here and restart blender. Then you can equip the proxy mesh under "apply assets" -> "topologies library".
As it is the same file format, you use the same procedure for creating additional proxymeshes, ie by using MakeClothes. As a hint, there's an almost finished new version of makeclothes under the "create assets" panel in MPFB but it's a bit rough around the edges still.