I asked one of their devs this question last November. Then newbler only used a single CPU, but they had plans to implement multiple cores in the future (but no firm date). I do not know of any developments.
In my experience, newbler (from version 2.3) manages to use a lot of the cpus it gets. However, cpu use is very dependent on the step in the assembly process. Read alignment is very cpu intensive, while building contigs/scaffold is a single cpu step. So, monitor cpu use and at the same time the standard output of newbler. might give you a better impression.