ImgBurn automatically booktypes DVD+R DL media to DVD-ROM on Samsung drives, and, on top of that, the drive should automatically booktype to DVD-ROM by itself.
In the 'Change BookType' dialog, select 'Samsung', then 'Change (For DVD+R DL Media)', then select 'DVD-ROM' and click the button 'Change'. Is it successful or not?
Also, where are you seeing that the burned disc wasn't booktyped? With ImgBurn in Write mode and that disc in the drive, can you copy and paste here the media information text shown inside ImgBurn's window?
By the way, you should install the latest ImgBurn.