Did you create a assembly.sbt at the root of your project? Alongside your build.sbt?
If so, then that’s the problem. You want to have it inside the project directory.
Having done that it worked out the box as expected with the rest of your setup:
> assembly
[info] Including: scala-library.jar
[info] Checking every *.class/*.jar file's SHA-1.
[info] Merging files...
[warn] Merging 'META-INF/MANIFEST.MF' with strategy 'discard'
[warn] Strategy 'discard' was applied to a file
[info] SHA-1: 1ae0d7a9c433e439e81ce947659bf05aa62a2892
[info] Packaging /Users/dnw/Desktop/t-2015-04-08.2340/target/scala-2.10/test-assembly-1.0.jar ...
[info] Done packaging.
[success] Total time: 2 s, completed 08-Apr-2015 23:45:59