Historical:How to install plug-ins in Photoshop
Simply navigate to the PhotoShop directory. For a Windows install, it's typically located at:
C:\Program Files\Adobe\Photoshop 7.0\Plug-Ins
If you installed PhotoShop elsewhere, or can't find it, do a search for "plug-ins" or "photoshop" - note that I am using Photoshop 7 in this example. Your's may be different.
Most plug-ins can simply be dragged or copied into the plugins directory. If you have several from a single source, you can create a subdirectory (folder) within the plug-ins directory and put them there. The next time you start PhotoShop, the plugins will be available, typically under the Filters option.
Some plugins have an installer, so you can skip the stuff above. However most don't!