How do I access an asset field in the 'onSaveEntry' event?
This gives you an EntryModel:

$entryModel = $event-&gt;params['entry'];


This will give you an array of AssetFileModels:

$assetFileModels = $entryModel-&gt;myAssetFieldHandle


You can loop through them like so:

foreach ($assetFileModels as $assetFileModel)
{
    $url = $assetFileModel-&gt;getUrl();
}