in this line:
getURL("/About Us.html", "_self");
do you have actual file with the title About Us.html?
The file name should be without breaks, so it should be AboutUs.html or aboutus.html or About-Us.html and so on.
though again, it seems that button self is not responds to getURL function anyway.
Again, read some tutorial on how to create buttons and write getURL for it. Try to create a button by yourself and get it working. With Flash you really need to repeat what you see in tutorials to get grip on basics.