I created one imageset that installed on an iMac very quickly from a MacBook Pro using Thunderbolt, although FireWire 800 was only a little slower. I deleted that trial imageset and attempted to create another essentially the same, but the 'Create Imageset' button is greyed out. I used an uninstaller to get rid of Lightning and then re-installed it. Same problem! Any advice would be helpful.
Please make sure you are using a 10.8 "Install OS X Mountain Lion" while using 10.8 machine. Drag the full .app into the box. Make sure it does not drop down into the second section. If you are still having the issue, can you upload a screen shot of the issue?
I tried a different Mountain Lion 10.8.2 Installer and that solved the problem initially.
The one I used originally was on a partitioned 64Gb USB 3.0 stick which I use for clean ML installs and had been careful to leave connected whilst the imageset was constructed the first time. The second time I used this same ML installer was when I had the problem, and I noticed that although I had placed it, as instructed, on the top section, it immediately dropped down to the second section.
I've now used a Mountain Lion 10.8.2 Installer 12C54 directly from the internal SSD, as are all the elements of the image. Unfortunately I seem to be getting various errors when the Image is trying to install onto the receiving test iMac, most notably iTunes 10.7 and iPhoto according to the log. If I try and create a new image the Installer on the internal drive slips down to the lower section.