Use a ssh client like http://www.chiark.greenend.org.uk/~sgtatham/putty/
Or directly with the firewall infront of you with keyb and monitor….
Connect to you firewall and login, choose option 8 (Shell)
Then
cd /cf/conf/backup
Now youre in the config back up directory, see what files are listed
ls -l
You should see a lots of config files named "config-xxxxxxxxxx.xml", look at the timestamp on those files, its for you to determine which file to copy based on when you fudged up the settings, if youre happy with your choice, run
cp config-xxxxxxxxxx.xml ../
The above should copy the config file to /cf/conf, we'll drop down to this directory and then backup your old config just in case, then we'll move the 'working' config in place of the old config thats giving you trouble.
cd ..
cp config.xml config.xml.backup
mv config-xxxxxxxxxx.xml config.xml
You should be done, just type exit in the console, the console menu should come up, then choose option 5 (reboot) sit back and pray.