Notice: This article may contain information for multiple versions of Passpack, be sure and read the section appropriate to your version. How do I know what version I'm using?
This depends on your browser settings.
In Firefox: Go to Tools > Options > Tabs > Links and make sure 'Load external links into a ' is set to New Tab.
In Explorer: read this article
In Safari: read this article
Passpack v8 (Preview)