You need to find a configuration file (or create one) that contains the list of available multiplexes and then drop into relevant location on your system.
This can actually be one of 2 places:
/usr/share/dvb/dvb-c <- check if its already there your system might have up to date files?
/usr/share/tvheadend/data/dvbscan/dvb-c
If you stick it in the first location you will need to go to the UI and edit Configuration->General->DVB Scan path and set to /usr/share/dvb
Adam