You can add a blue tick mark at a location a by typing an expression in the "Value" text box. The syntax is standard. For example, try typing in turn
If you wish to save a point and its label (if it has one), click on "Add Label" before going on to the next number. The "Clear Labels" button will clear off any points and their labels that have been added.
Here is an interesting demonstration that uses the decimal scale and adding numbers features. Entering pi in the "Value" box will locate a point at π. Turn on the horizontal ticks and slide the line along until you can see the mark at π. Add the label Pi, say, at π. Now add a point at 22/7. The label 22/7 will sit right over the label Pi. Now zoom in step-by-step and watch the points resolve and move apart as you view them at smaller scales.
The mathlet also recognizes e as the real number e.