Oracle has recently complicated VirtualBox. They now have only one version
and Extensions that you installed to recreate what was in PUEL (usb support,
etc.). The Extensions must be downloaded and installed separately.

The step for using VB is to install it and then you need to open Users and
Groups in the Systems menu. Then Unlock which gives you admin access and
check to VBoxusers group at add you to it. Now you can use VirtualBox and
create VMs.

Installing the Extensions involves downloading the one matching your
VirtualBox version, then right-clicking on the downloaded file and choose
Open. Then navigate to Oracle VirtualBox in the menu under System (varies
with Ubuntu version or Kubuntu). You have to agree then it will install.

Finally once your VM is installed and running you still need to add the
Guest Additions. You can do that from the VirtualBox menu. It will set it up
as a dummy CD drive and install from there.

It sounds more complicated than it really is. It now has one version instead
of two but you need the Extensions to do what the PUEL version did. You
always needed to add yourself to the user group and install Guest Additions.