This one doesn't seem to work for me: http://www.unifycommunity.com/wiki/index.php?title=BuildAssetBundlesFromDirectory
What error is showing in the console? Do you have Pro version?
The problem is here, missing an 's', i.e. should be Assets, rather not Asset in the line: [@MenuItem("Assets/Build AssetBundles From Directory of Files")]: http://unity3d.com/support/documentation/ScriptReference/BuildPipeline.BuildAssetBundle.html?from=AssetBundle
The link I referred to in my original above message is actually OK: ttp://www.unifycommunity.com/wiki/index.php?title=BuildAssetBundlesFromDirectory
These 2 links present very similar contents, just that one by unity3d.com is the faulty one.