If you don't see a boundary chances are there was no boundary added to your field. You can go into SWAT RECORDS desktop and add your field boundary shapefile in the field region. See adding a field instructions for more detail.
Note: All new SWAT MAPS should have the boundary added already. For all legacy fields, you can find your boundary shapefile when you go to add the boundary in field regions when you click on add a region. It will open to the fields Maps Data folder if you've File Synced.