Tammi, I too have had issues with this and the video etc and help so far is very poor.
What I did is a first put the kml file on my web server so that it was available as download. Once I did that the kml loaded as per the tutorial.
However, whether it will load or not depends on the kml layers involved and if too many layers then it simply will not load and appears blank. What is clear from my experience is that the structure of the kml file and the number of layers appear critical.
If I copy the kml example that WHATSAPP provides it works fine and shows the state boundaries fine.
However, if I export a kml file from say Google Earth it will not display. It appears support always skirt this issue but I feel it is the structure of the kml file that is very NB.
Other than that I have no desire to chase shadows but would guess that keeping the KML file structure very simple will work fine. What the breakpoint is I have no idea only that simple works complex not.
Appsheet needs to more clearly define this complex breakpoint as they are offering the feature.