kernel version mismatch - feels like it doesn't like the 64bit mod. never forget that slamd64 is an inofficial port of slack. try installpkg -q <packet> to mute the warning messages.

otherwise build it (the whole application) from source. the source is available as well as build instructions from the virtualbox website. it's customized for debian though, but it's a pretty much standard compilation with ./configure, make and make install.

if you can't get virtualbox to work, try QEMU, Bochs or Xen (though i have no experience with them).