I can't reproduce this, the -fx-font-weight:bold style works fine for me when set either in code via setStyle or in the exact same external stylesheet you supplied (WinXPsp3, JavaFX 2.1b17, JDK7u4ea).
My guess is either:
1. It is a Mac specific bug.
2. The default font used by JavaFX on the Mac does not support bold (seems really unlikely).
3. You are not copying the correct version of your stylesheet to the binary output directory you are running the application from (most likely).
4. You are not correctly loading the required stylesheet.
5. You are otherwise overwriting the style somewhere else.