It sounds like you are running PHP4. You should be able to use hungryfeed on PHP4 however you have to comment out some lines.
comment out lines 328 through 331 in hungryfeed.php which are these lines:
try {
$source_title = $item->data['child']['']['source'][0]['data'];
$source_permalink = $item->data['child']['']['source'][0]['attribs']['url'];
} catch (Exception $ex) {}
I think that’s the only place that is PHP5 specific. disabling those lines will only affect a minor feature of the templates to grab the “source” fields which most people don’t use anyway.