I'm getting the same thing - maybe it worked that way in a previous version and they haven't updated the help file. I was able to do it a different way: make sure the two planes are inactive, select smart dimension and select the two planes. When one plane is active like it says it should be in the instructions then it doesn't work, but when both planes are inactive it works.
Yes I think so too. The help is not upto date. It infact got me confused and made me spend more time on this than required.