I recently ran into the exact same permissions problem with wordpress on Ubuntu, so its ubiquitous on self-installations.
The www process needs to be able to write to the wordpress contents directory, or wordpress can't work. I found at least a dozen walkthrus on the web about how to fix this...they mostly use the linux www user:group names, so sub in the macos names. when you sort it out, maybe consider writing up a wiki on it, as its a commonly-used piece of software many might want. K
