In the wizard, click on lights. At the bottom of the window you will see update, cancel, and add device. Click add device. A lighting device template window will appear. At the device template dropdown select the template that most matches the device that you are trying to set up. It will either be a dimmable device or an on/off device. Click get device template. Then fill in the data of the device that appears. In the Port/channel text box enter the house code/device code address of your x10 device: A1, B12, G15, etc. It will be one letter and one number. Fill in the other info if it is relevant. The description text box and the room drop down are the only other things I usually bother with. I have not attempted to do floor plans yet so I cannot help you there.
I hope this helps.
Also, there is a lot of info in the wiki. wiki.linuxmce.org. Some of it is dated but most is still relevant.