Add Assets to Bundle Using Only Scripting

Hi, I'm a little confused as to the implementation of AssetBundleBuild[]. (I'm very new to using asset bundles.)

I'm trying to build an asset bundle from script only (eventually, the final program should download files off a web server based on a url input and then input them into the bundle). I tried the following code with a locally saved file, but I don't know how to run it automatically/triggered through script as it is an editor script, which is apparently necessary to build asset bundles:

 AssetBundleBuild[] bb = new AssetBundleBuild[1];
 bb [0].assetBundleName = "btnmedia"; 
 bb [0].assetNames [0] = "Assets/Media/cat.png"; 
 BuildPipeline.BuildAssetBundles (path, bb, BuildAssetBundleOptions.None, BuildTarget.StandaloneOSXUniversal);

Thanks!