The -D command sets up a new property throughout your build.
So, in your command line, when you type:

ant -Dant.dist.dir=directory_to_contain_Ant_distribution

Thoughout your build you can access that directory by an Ant property such as ${ant.dist.dir} which will have the value of the directory specified.
You do not have to create the new property in your build by the tags since its already created.
Let me know if this helps or not!