Depends if you want the open source edition (from SlackBuilds.org) or if you're happy to have the closed source version. If it's the latter, just download the Generic Linux installer from virtualbox.org, and run it as root. It'll install into /opt by default and will build the necessary kernel modules automatically, so long as you have the kernel source installed for your kernel.
I've no experience of the SlackBuilds.org version, but can't imagine it'd be too difficult so long as you read the accompanying README
edit: If you're running pure Slackware64 (i.e. NOT multilib), then the OSE VirtualBox build from SlackBuilds.org won't build. You either need to install the 32 bit compatibility libraries, or just use the close source version from VirtualBox.