Sounds like you are trying to create a Domain in your output geodatabase, but that one already exists with that name. The "DEF line" is an item in the underlying mapping file. It's parameters are exposed in the Feature Type properties dialog (click the [...] button on the top right of the output table name). You'll probably want to check whether you mean to add to or overwrite the geodatabase and similar settings, especially the domain ones.
I'll ask the developers to change that error. We shouldn't really mention DEF lines these days.
In the Format Parameters, I have it set to the following:
Table exists: Yes
Truncate Table first: No
Drop table first: No
All the rest are greyed out.
I'll ignore it for now.
Thanks,
Sean
Just to mention the error has been updated in 2013-SP1 (beta build 13433) to be a bit more useful and not mention DEF lines.