LaunchDaemon won't run

Sorry if this isn't the most correct place for this thread. Since it has to do with updating dynamic DNS via a script I thought DNS would be as good a place as any. So I'm using a script called zoneclient.py to keep my DNS up to date (If I can get it working reliably I'll save $50 per month on my Internet bill by killing my 2nd static IP line coming into my home). The command I'm calling is: /usr/bin/python /Volumes/Data/Users/david/Documents/zoneclient/zoneclient.py -q -d /Volumes/Data/Users/david/Documents/zoneclient -r http://getip.com tknospdr zone-edit password technospider.com,mail.technospider.com This works interactively via Terminal, and I'm pretty sure it's working as expected via cron as well. My problem is I want to do it the "correct Apple way" and have it running via launchd. So I created this file:

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
<plist version="1.0">
<dict>
	<key>Label</key>
	<string>com.david.zoneclient</string>
	<key>UserName</key>
		<string>david</string>
	<key>ProgramArguments</key>
	<array>
		<string>/usr/bin/python</string>
		<string>/Volumes/Data/Users/david/Documents/zoneclient/zoneclient.py</string>
		<string>-q</string>
		<string>-d</string>
		<string>/Volumes/Data/Users/david/Documents/zoneclient</string>
		<string>-r</string>
		<string>http://getip.com</string>
		<string>tknospdr</string>
		<string>AAZ-Bd3-4Xe-qox</string>
		<string>technospider.com,mail.technospider.com</string>
	</array>
	<key>StartInterval</key>
	<integer>900</integer>
</dict>
</plist>

But this is NOT working from what I gather. Anyone see an obvious fault in this script? It's really bugging me that I can't get this working properly.