It's likely that the graphical settings interface simply can't support more than one pointing device. Therefore, you may need to add the device and adjust its settings in xorg.conf. Check out the documentation on how to do that here:
Once you've defined the device, it looks like you can use xorg.conf's "Sensitivity" option to adjust it, or use
They don't specifically go into it on that answer, but
man xinput shows that
[DEVICE] can be one of the primary arguments to xinput.
In your question, you say you have both a mouse and trackball plugged in. In your bounty comment, you say you need a solution because you only use a trackball. If you unplug the mouse and only plug in the trackball, then Mint's graphical Mouse configuration in the Settings panel should work just fine to configure the trackball sensitivity. (Where it gets tricky is defining what your trackball buttons do-- emulating scrollwheel and middle click behavior, if necessary, but that's not what you asked about...)