In Cartographica, each feature (point, line, or polygon) exists within a layer of similar features. To get started with drawing a new polygon (or point or line), you first have to make sure you have an appropriate layer. If you alredy have a layer that you would like to add the feature to, skip directly to step 2. If not, create a new layer in step 1.

The result should be a new layer in the Layer Stack to the left. This layer should already be selected.
Note that if this is the first layer in the map, you will see a warning triangle because there will be no Coordinate Reference System (CRS) set for it. You can determine the CRS by choosing Layer > Set Layer's Projection…

If you have just created a layer, it will be selected. If you are using an existing layer, select it by clicking on the layer in the Layer Stack.
If you are adding a feature to an existing layer, skip the next step

Choose the type of feature to add: Polygon, Line, or Point

You are now in feature edit mode. In this mode, the existing map and features are dimmed to allow you to better see what you are doing.

The feature is now visible on the map. The layer stack also indicates the type of feature that you have created.