You probably have to switch to root.
Well I am surely not a Linux expert, but what I do is issuing "su" command (without any options) before editing dhcpd.conf, which gives me the root access.
I have stored dhcpd.conf into HD linux partition and called everything with a small script I made and put in the same partition.
The script initialize my network card ("ifconfig" specifying IP, etc) and calls dhcpd3 (thus NOT dhcpd!) command with "-cf" option, which specify the new location of dhcpd.conf file.

Don't forget to unlock write permission on your Linux HD mount. Right mouse click (over the partition) gives you that possibility.
Success.