Only the MCE version of Movie Maker supports DVD's at all and that version
uses Sonic MyDVD to burn the DVD's. I can't say for certain if MyDVD
supports DL disks or not but regardless the last thing you want to do is
burn directly to a DL DVD from Movie Maker. The discs are far too expensive
and burning directly from Movie Maker to DVD tends to turn blank DVD's into
coasters. I recommend investing in a decent DVD Authoring program and using
that to create DVD's. Most of the good ones, Ulead Movie Factory and DVD
Workshop included, do support DL discs. I have a list of the more popular
programs on my website in the "DVD Creation" section.along with download
links and some How-To's on the use of a couple of them. Most offer a free
trial so you can decide which you like best before spending any money.