For my VirtualBox 5.0.14, I downloaded and installed the Extension Pack (All Supported Platforms) and I also became a member of "vboxusers". From what I have read up on the matter, these ought to have sufficed for the Windows XP SP3 running in the VM to detect a USB device but this is not happening.
A few more details:
Originally the USB Settings in the VM allowed only USB 1.0, but now it allows both USB 2.0 and 3.0. This I think is sufficient proof that the extension pack has been properly installed.
For becoming a member of the vboxusers group I used the command "sudo apt-get genome-system-tools" in the terminal and then opened "Users and Groups" from the dash. I then clicked "Manage Groups", found and selected "vboxusers" from the list and clicked "Add". At this point, however, I was presented with a choice I didn't fully understand: I was asked to provide a Group Name once again! So once I entered just "VirtualBox" and at the second time "Windows XP" (my guest OS).
I hope someone more experienced can help diagnose the problem and suggest a remedy.