Hello, if i draw a curve, I can set a marker (an arrow for example) at the beginning or the end of the curve. But I want it to have it somewhere in the middle. How can I achieve this. The "middle Marker" does nothing. Thanks.

How many nodes do you have on that curve? Markers appear where the nodes are on the curve. You can use Align option to equally distribute the nodes so the markers will be equally distributed too.

If you don't have enough nodes on your curve add more using Edit nodes tool