Dear Mark. I have installed the mediawiki 1.21 and I am testing it. More comments about installing:
1. Setting of default host is tricky: one needs to guess, that for OS X, the correct value is 127.0.0.1. (as for Windows)
2. I think, the default setting should include the most popular extensions. You may consider to include Typing of formulas, Use of images (including *.pdf), and Citing of references into the default installation.