If you install gksu (search LQ
for how to find and install programs in FC3) you should be able to do the following.
1. Put the command (ppd call ac555-1x) into a script somewhere in your home directory (~/bin, as a hidden file, etc.) and make that script executable.
2. As the command to launch in the launcher put 'gksu scriptname' where scriptname is the full path to the script in step 1.
Hopefully this will work for you.