I have found a work around the issue, un UE 5.2.1. If the metahuman does not appear automatically after clicking add from the Quixel, just open the content browser below, you can notice there is a folder by the name of the metahuman you have just added. Click on the folder and click&drag the metahuman into the level. Thats it, your metahuman will be loaded to the level editor. You can start editing. Happy ending.