Connectors are largely broken in Calligra Flow 2.4.2. For one thing, the clickable area in a connection point to create a connector is very small. At the default zoom, the area appears to be about 3x3 pixels here. It can increase a bit with more zoom. However, it's also variable depending on the point. For example, on the side points of a pentagon, the area appears to contain a single pixel.

When one succeeds to hover that area, the cursor will change into a long arrow with a connector symbol to its left. However, for rusty or new users, the feature will appear to be unusable.
Note: if hovering the zone seems absolutely impossible for you, try clicking somewhere else (in an empty zone) and retry.

Reproducible: Always