Answer courtesy of C. LePard, DXC Technologies
I am unaware of any issues adding either MDSs or modules to a tree; this is an action that is performed literally thousands of times a day. It may be browser or cache related. Google Chrome and Mozilla Firefox are the best browsers for IMDS use, and Microsoft Internet Explorer and Opera work OK. Microsoft Edge does not work well; if you are using Edge, you may wish to change browsers. If you still have issues, you may want to flush your cache and try again. If you still have issues, please contact the Helpdesk.