Troubleshooting radius is a crappy task. But crappy articles don’t help:
http://support.apple.com/kb/HT3929
To be more specific, the debug mode flag is -X (not sure why that was so hard). In that case it’s doing single server mode and the process cannot fork. You can also do the lowercase, -x (which is part of -X), or -xx for further granularity. In order to set the launchd item to debug mode you would therefore find the /System/Library/LaunchDaemons/org.freeradius.radiusd.plist file (only created once you’ve fired up RADIUS btw). From here, locate the array for invoking the command:
<string>/usr/sbin/radiusd</string>
<string>-sf</string>
Change the -sf to either a -X or add an x or two in there as needed. I’ve also had to enable core dumps for troubleshooting RADIUS as well, which means editing the /etc/raddb/radiusd.conf file, looking for allow_core_dumps and changing it to an = yes instead of an = no. Anyway, just finishing their article for them as my own little core dump to you.
