The first step is to ensure that the lines making up your boundary are on the two layers indicated in Properties for the Physical and Access layers. By default, these are PCPHYSICAL and PCACCESS, but you can set them to whatever you require - just ensure your lines are on the layers you have specified. The second step is to ensure that the boundary lines either meet (i.e. using the AutoCAD® drawing tools: Fillet of radius 0, Trim, or Extend) or cross (i.e. the endpoints extend beyond the meeting point).