You need to be an administrator on your machine in order to edit the routing table. The IT guy you worked with switched to
root which allowed him to do this. If you don't know the root password (and he won't tell you) then you can't take that path.
However, if your account is in the admin
group then you can use
sudo to temporarily elevate your privileges to run specific commands. It's worth a try, no matter what - just precede the command you want to run with
sudo:
<pre class=command>sudo route add -net 1.2.3 10.9.8.7</pre>
You'll be prompted for your password - which is
your password on this machine (to make sure it's really you and not just someone using your system). If all goes well it'll work, otherwise you'll get a 'not authorized' error message, in which case its back to the IT guy for help.