You are supposed to unpack everything into QB64's main folder, so that you'll have UiEditor.exe and an InForm subfolder sitting in the same folder as qb64.exe. UiEditorPreview.exe isn't supposed to be run by itself, as it'll be called from UiEditor.exe.
Make sure you tell your AV software that these are safe to run (if you prefer to, you can delete the binaries and recompile UiEditor.bas; then, when you first run it, UiEditorPreview.bas will be compiled automatically).
When run, it'll likely take a few seconds to show the main interface, but you'll at least see "Initializing..." or "Launching...". If you didn't see either, try grabbing the latest build of QB64.