If that map was correctly made, it should become visible when you enable custom maps on your Oregon. The setting is global - all custom maps you have installed are either on or off, you cannot turn individual ones on/off.
You need to create a folder named CustomMaps inside the Garmin folder on your GPS (or a data card) and place the file inside of it or the unit won't recognize it. There are also some strange issues where certain maps don't work because of the filenames. Filenames can only contain letters and numbers, with no whitespace and now special characters. Further, .kmz files are actually zip archives that can contain many individual .jpg files. If any of those .jpg's has an illegal name, it can also cause the map to not be recognized.
You need to zoom WAY in to see custom maps. The zoom level varies depending on the detail (resolution) in the map images. Zoom in to about 300 feet and make sure you are centered on a location inside the park and it should work if the map was correctly made.
On my Oregon I've seen a stange bug where custom maps sometimes don't show when I first turn them on. Then if I close the map view, go back to the main menu, then return to the map and drag it around it will show. I have the older Oregon 400t and haven't upgraded firmware for awhile, so I don't know if your 450 will have this kind of issue.