hey Edd, bitsetting function is not in the control panels. if you have installed Plextools Pro, open it up and select your burner from the upper left pane. then go to Drive Settings and click on Advanced tab. at the bottom there are checkboxes for set booktype for DVD+R and DVD+R DL. make sure they are both checked.
depending on what program you are using to burn the reauthored disc, you may need to set the booktype from within your burning program.
in Nero, go to Choose Recorder and there is a way to set the booktype there.
if you're using DVD Decryptor to burn the disc, where you see the Burn speed drop down menu, there's a little icon that looks like an open book. click that and use it to set the booktype to DVD-ROM (make sure you have the most recent version of DVD Decryptor as the ability to set Plextor drives' booktype has just been added).