Kind of an unusual Ubuntu 12.04 system. It was built up from a minimal install and doesn't have most of the usual Ubuntu graphical elements: no Unity, no Gnome shell, no LXDE, and so forth (I use the i3 window manager). So directives like "go to System > Utilities > . . ." won't apply to a system like this.
Furthermore, I've been using an old serial mouse on this system--a Kensington Expert mouse (trackball), which I really like from an ergonomic perspective. I've currently got it hooked to the system via a serial-to-ps2 adapter and it's been working acceptably for the most part--including the crucial copy/paste-via-middle-mouse-button functionality--though some odd quirks have cropped up lately. As a way of possibly addressing those quirks, I've decided to try getting the serial-to-ps2 adapter out of the picture and hooking it straight to the serial port.
So I found directives for getting a serial mouse functioning under Ubuntu (https://help.ubuntu.com/community/SerialMouseHowto) and, implementing those directives and hooking the mouse to the serial port does result in a mostly working mouse. The problem is that one of the functions I use most and absolutely must have is not working under this scheme: I can, holding down the left mouse button and dragging, copy text from graphical or terminal programs, but I cannot paste it by clicking the middle mouse button at the desired location. In short, there seems to be no middle mouse button functionality present under this schema. So I'm looking into ways of possibly restoring that middle button functionality.
I should mention that, of the possible inputattach options, none are a very close match for my mouse. I tried several and some result in a working mouse, while others do not. I tried initially --microsoft, but have also tried --mshack and --mouseman. With none of those do I get any sort of middle mouse button functionality.
xev show, by the way, when the mouse is hooked to the system via the serial-to-ps2 adapter, that I have (starting from lower left and moving clockwise around the mouse) buttons 1, 2, 9, and 3 on this mouse. I suppose button 9 would be a good one to assign as a middle button. Or perhaps a combination of 2 and 9. But when I remove the adapter and hook the mouse straight to the serial port, xev indicates only 2 buttons are present: both upper and lower left buttons report as button 1, while both upper and lower right buttons report as button 3.
So, do I need perhaps to do some mouse button mapping? There are 4 physical buttons on this device. Perhaps I will need to put some supplementary xorg stanza in some /usr/share/X11/xorg.conf.d/ .conf file? Suggestions will be appreciated.