In order to wire two blocks together, it is best to place them side by side so that the source block instance is displayed on the left and the destination instance is to the right of it.

Then point to the output wiring point on the source block. Note how the mouse cursor changes. Press the left mouse button, and while holding it down, drag to the input wiring point on the destination block. If a connection is possible the line being dragged from one wiring point to another will turn bold to indicate that you can now release the mouse button and create the wire.

If you release the mouse button elsewhere, and when the line being dragged is not bold, then nothing will happen. You can try again.

If you release the mouse button correctly at a point where the line can be created, then the Configure Block Wiring dialog will appear.