Top right in the InvokeAi webui, there's an isometric cube icon for "Model Manager".
Click that, you'll see "+ Add New".
Right click in the relevant Windows folder path containing your model(s), and select "Copy Address".
Ctrl-V to paste into the "Checkpoint Folder" field and hit Enter.
You should now be able to add your model of choice, and all going well, it should then appear in the drop down, to the left of the Model Manager icon.
Hope that helps ;)