Have you tried Setup Diagnostics in the Scala menu?
Then check your Project Properies, under Scala Compiler, check that the Use Project Properties box is unticked (so that you will use the global defaults).
Make sure you have your project set up as a Scala project rather than a
Java one. If not, you can add a Scala Nature by right-clicking and selecting Configure...
I found the Eclipse error messages were not very useful. If your project doesn't compile, it tells you it had a problem loading your classes, rather than giving you the compiler error message.