This command actually works on faces and not subD objects.
1) Right-click over your subD object and select Faces.
2) Select the faces that you wish to apply the mapping to.
3) Now go back up to Subdiv Surface->Texturing->Planar
I stumbled across this in the online help docs. Go to Help,
search for "Map UVs onto a subdivision surface", select the first tutorial, - the one whose name is the same as the text you just searched for. That's where I got the info.