You won't see the dimension as you are selecting the points.
When you finish selecting, you double click anywhere and you will see a selector, where you can choose vertical, horizontal or angled (if you chose that dimension type) and the position where you want the dimension to be.
Once you have placed a dimension you can select it and add nodes and you will see the changes as you make them.
Barry.
One of the forum moderators.
Versions 6.5 to 27
i7-10700 @ 2.9Ghz, 32GB ram, GeForce RTX 2060 (6GB), Windows 10
Lenovo Thinkpad - i7-1270P 2.20 GHz, 32GB RAM, Nvidia T550, Windows 11