Yes, that is a problem, especially with larger objects - I would still say though don't let that not let you contribute.
If you are interested in changing the underlying terrain, such as the improvements here:
http://mapserver.flightgear.org/map/?lo ... FFTTTFTFFF
the proper free program to do this is probably QGIS, though there are others available. That runs on a Mac. There is a learning curve.
If you're interested in improving San Francisco, I have a shapefile of various parts of the Bay Area partially complete which I can post.