Mosent recent version is at puppy/opt/, supports Puppy 4.3.x and most of the major Puppy-5.x.
virtualbox-4.1.6.sfs is for Puppy 4.3.1 and most of Puppy 5.x. It does not support Slacko. Use virtualbox-4.1.6-slacko.sfs as for the Slacko 5.3/5.3++.
If you have ever installed another or older PET/SFS and trying new one, remove all the old ones and delete '/lib/modules/*/misc/vbox*.ko', if they are exists, before loading the new.
For some kernels, you need devx and kernel source packages to build the kernel modules. Type 'virtualbox' on a terminal after loaded the virtualbox-xxx.sfs.
Visit Puppy Linux Discussion Forum, the topic: VirtualBox for many Puppies for the detail.