check what version of debian you are running:

uname -a; lsb_release -a;
Linux debian 3.16.0-4-686-pae #1 SMP Debian 3.16.7-ckt25-2+deb8u3 (2016-07-02) i686 GNU/Linux
No LSB modules are available.
Distributor ID: Debian
Description: Debian GNU/Linux 8.5 (jessie)
Release: 8.5
Codename: jessie

for jessie proceed:

mkdir -p /software/virtualbox
cd /software/virtualbox

apt-get update;
apt-get install libssl1.0.0;
apt-get install libqt5x11extras5;
apt-get install linux-headers-$(uname -r); # install linux header files
apt-get install gcc make;

dpkg -i virtualbox-5.1_5.1.2-108956~Debian~jessie_i386.deb; # install

/sbin/vboxconfig; # you might need to rerun this to build and install virtualbox kernel module Starting VirtualBox services. # these two lines should be the only output Building VirtualBox kernel modules. # you do not need to restart

now copy all the VMs into place (were they used to reside like a folder /VMs) and double click a *.vbox to add them to the VirtualBox-VM-Manager-GUI.

Should work.