You should have a build area ready before importing the Ant buildfile. The base directory of the Ant buildfile should correspond to a Dimensions work area.
Also consider using Dimensions deployment areas for some or all of the directories. For example, if the buildfile refers to a \dist directory, that would be a good candidate for a deployment area.
If it is necessary to create a new build area, remember to attach the work area. The procedure for doing this is described in Build Areas.