To generate surface meshes from the contours in your DICOM RT file, choose Generate Meshes from the main menu. Here you can choose which contours you would like to generate meshes for. And previously generated meshes are overwritten, but it is quick and easy to regenerate them if you need to.
Tick the boxes next to the contours you want meshes for, and click on generate. When this is finished (it only takes a few seconds), you can close this dialog, and see and interact with your meshes in the 3D viewport.