You have to pair the Vera/RFXtrx with your light switch, exactly as you will do with a remote. There is certainly a process to do that in LightwaveRF.
For the unit code, I don’t remember for LightwaveRF if it is hardcoded in the light switch or not. If it is, you have to use the unit code of your light switch when creating the Vera device.
To pair the Dimmer with out using a remote you just need to go to the RFXtrx plugin under Devices screen and you will find a button that says “Add LWRF Dimmer”. Before you click that you need to give the light switch a hex value and a unique id for the switch (01 - 16).
Once you have giving it a hex value (think of it as a remote control ID) and a device ID number click the button “Add LWRF Dimmer” and put the switch into pair mode. It will Pair.
Note: If you have paired a dimmer by a remote, take a look a the dimmer in Vera under devices and you will see it has a hex value and a 2 digit ID. From this you can figure out the format and take the remote back to B&Q (if it was not in a frikin blister pack you have to damn near destroy to open)