It's likely that your running some font management software on your computer and/or have Helvetica active. Apple has a Helvetica font pre-installed on your system, so any other Helvetica that you try to activate is causing a conflict. If you need to use Helvetica in documents you're designing for print, switch to using Helvetica Neue for your design purposes and remove the Helvetica in Suitcase/Font Book or whatever your using.