I have no idea what you are doing, as you are telling me a story about what you think you've done, not showing me exactly what you are seeing. The commands given do work, as they are a copy/paste directly from my own terminal, and the two commands are in fact entered into /etc/rc.d/rc.local. My rc.local now looks like this; (Note the last two lines)
[root@fatman ~]# cat /etc/rc.d/rc.local
### BEGIN INIT INFO
# Provides: rc.local
# Default-Start: 2 3 4 5
# Short-Description: Local initialization script
# Description: This script will be executed *after* all the other init scripts.
# You can put your own initialization stuff in here if you don't
# want to do the full Sys V style init stuff.
### END INIT INFO
# Fix linkage to cdrom/dvd and permissons since udev is screwed
ln -s /dev/sr0 /dev/dvd > /dev/null 2>&1
ln -s /dev/sr0 /dev/cdrom > /dev/null 2>&1
chmod 777 /dev/sr0 > /dev/null 2>&1
chown root:root /dev/sr0 > /dev/null 2>&1
rm -f /dev/modem
ln -s /dev/ttyACM0 /dev/modem
If you run the above command, then copy/paste the prompt, command, and results to your next post, we'll see what you have actually accomplished. From the error message concerning having no such file, you obviously typed the command wrong, because the file was just found for the previous command.
As to your comment about not getting a $ symbol, that's an integral part of the terminal prompt for a normal user, so I have no idea at all what you are doing or where you are doing it. You need to be much more precise in your explanations.