You may or may not have heard or read that Autodesk has added BIM 360 Glue integration to BIM 360 Field. What that actually means is that in order to integrate a new model into BIM 360 Field, you NEED to use Glue.
If you have an existing model that you've already integrated into BIM 360 Field with the Navisworks plug in and you need to download/install the plug in for a new install of Navisworks, it's still available. If you go to the "Setup" link in BIM 360 Field, then choose "Equipment", then choose the "Models" tab, which is where the link has always been in the past, you'll see the page shown below.
If you click on the "Learn More" link you'll get more information about how to integrate a BIM 360 Glue model into Field.