Hi Jens,
You don't specify which Debian 7 version/arch you're running there, so I'll ask: Are you running one of the kfreebsd versions as your guest? I doubt we officially support those at this point, and I wouldn't be at all surprised if Tools refused to install and died of utter confusion and embarrassment.
If you're just using the regular amd64 build, just switch the guest OS selection to Linux > Debian 7 x64 (or whatever is the most recent Debian x64 included on the list in the version of Workstation you're using) and it should install the right Tools package.
Cheers,
--
Darius