Okay, I figured it out. We go into the "shared" section...
We make "possible tiles." I copied these coordinates from the "blocked tiles" and then deleted them. Now, we can walk on the log, but we're still standing knee-deep in water. =) (Also, set "hero trace" to "true" – I don't know what that does.
Now, click on "model" (the first line in the screenshot). There, find "geometry" and go to the object's geometry. I'm sure there are many interesting things there, but we're interested in "best fit point." I have no idea what it is or what it's for. It was at ~0.3 when I set it to 4 – and, lo and behold, the hero walks across the bridge. I hope this helps. Although, this is all very new to me too. ;)