What version are you on? You shouldn't be able to add a new role at that part of the tree - only at the branches below where you'll also be specifying the type of role it will be.
We are on version 7.2.6.
Sometimes I get this error when I try to create a support group, a user, a Window, ...
Have you tried to close LDSD Console, run a iisreset and re-launch the LDSD Console ?
Generally, it works for me.