Slitherine

I would like to map my extra mouse buttons to next unit and previous unit, but it doesn't accept mouse buttons.

Please consider adding mouse button mapping in an upcoming patch.

Until this gets implemented into the game you can use XMBC to help yourself out. It's a little free tool that intercept mouse button presses and can change their behavior. XMBC will work with most mouses but may not support all of the special keys on some of the exotic gaming mouses but they usually got their own software to do that anyway.

As how to use it: if you want to use your mouse button four to trigger a function mapped in the game to key "a" set the mouse button in question in XMBC to send the letter a. XMBC can detect which windows the mouse cursor is inside and only apply the change in a windows you want to so your mouse will behave normally in other programs.

I'm using the Logitech Gaming Software for this. Maybe there are also similar utilities for other mouse brands.