OK, so it's worth clarify that these are 2 different things you're trying to accomplish.
Server Roles
This requires a cross-forest trust, and uses Windows' AD membership to find and add the administrator (i.e. it's possible to add roles to admins without populating the AD section of the SEEMS Config Manager, it uses what Windows can find due to its AD membership).
Add AD Forest
This just give you an overview of the client machines in the target forest or domain that you add, and aids in client tracking. This doesn't sound like what you're after anyway, so you might just ignore the errors (which are likely occuring because nothing in your nomal AD forest knows how to connect to this new one.) If you sort out the cross-forest trusts and resolution, this should start working as well