To find this tool, do the following (see images):
* Click your Content Browser
* Click the Stock Tool Catalog

* Click Helper tools

* Click AEC Polygon tool eye dropper

Then, fill the eye dropper up, drag to model space and pick points to outline the area you want to hide (similar to picking points for a wipeout). You can edit the AEC Polygon style to set specific things you want it to do such as using a Background Mask, telling the edges to be set to a no plot layer, etc.
Its a fairly simple Style to control in ACA since there are few options for it, but should greatly simplify your creation of areas to mask.