It's been a long time since I used dialup, but I seem to recall doing this:
Open a terminal and enter the command:
Then Gnome-PPP may be able to see it.
sudo ln -s /dev/ttyACM0 /dev/modem
If it doesn't work, try this:
When you get to the configure modem part, answer "no" (to having the modem identified automatically) and enter the modem string yourself. That will be either /dev/ttyACM0 or /dev/modem (if you've used the command above).
Once you've finished, you can try to connect with the modem using "pon" to connect, and "poff" to disconnect.