I've just updated my unofficial Mac OS X build (featuring alloy's native sidebar with a file tree):

https://github.com/rcarmo/macvim/releases/tag/7.4

It's a tad controversial, I know, but I find it vastly more friendly than NERDTree, and I know quite a few people were looking for alloy's builds a while back.

EDIT: screenshot, from an older build:

https://f.cloud.github.com/assets/392683/793013/469e0c62-ebd...