So I've got a Kensington Expert Mouse, a mouse with 4 buttons.

I've installed the software that comes with it to activate the full configurability of it, but this seems to have given Dosbox a bad time of it.

It flashes an error before scramming along the lines of 'SDK invalid device configuration' or something to that degree.

Do I need to use another mouse, or is there something I can do about this?

I don't know of how much use will this be for you, but as far as I know you can't do much about this other than switching mouses. Maybe the software you installed came along with drivers that "confused" DOSBox completely, Have you possibly tried using it with that software not installed (I know this isn't the desired preference, just asking for diagnostic purposes)? If by any chance mouse works under DOSBox without it and you ascertain that said drivers are the problem, maybe you can briefly disable them when needed and reenable them later on.

DOSBox is an excellent software but it can't boast with too many pointing devices config options, especially with such exotic specimens as the one you have. But if you think about it, you would be having hard time making that mouse work as intended in DOS either, at least when it comes to advanced functionality :)

I'll try that. Thankfully we've got spare mice around, but I've just grown to prefer trackballs. Nothing quite like not having to move your mouse,

I use the same "mouse" and never have problems with DOSBox or other games. Of course I detest mouse software and have never installed any version of it.

I was disliking that button upper right was back. That caused trouble. :B