In order to start meshing in River2D program, you will need to download and install R2D_Mesh. It is an application developed by University of Alberta that allows you to generate meshes. Check the official user guide for further information. Unfortunately, you can't download the application from the official web page because the link is not working. I suggest you use
email@example.com email address to contact the developers. Also, you need to know that you can't import AutoCAD files in River2D.