Have the same problem on Win7 x64. Had similar problems on Vista x64 that could be solved by (shameless plug alert) using the steps detailed in this post on my site:
http://seanmonahan.org/2009/06/12/bl...-vista-64-bit/
However, I have not found a way around this problem with build paths on Win7. Looked at the various meta files for the project an couldn't get things added that way either, though I could add things to the library path....which just reminded me: this all works fine on Mac OS.
In the .actionScriptProperties file on Mac OS I have the following that correspond to me build path entries:
<compilerSourcePath>
<compilerSourcePathEntry kind="1" linkType="1" path="properties"/>
<compilerSourcePathEntry kind="1" linkType="1" path="locale/{locale}"/>
</compilerSourcePath>
Willing to bet that will solve the build path issue. At least in the sense that you can add stuff. Will try when I get home.