Hi Macindows, I was just searching for a solution to the problem that you are having and after solving it I felt compelled to share with you the simple solution. You need to enter the key stroke that the finder uses in common with your web browser, so here it is. Usually on your mouse in Steermouse the buttons are called 4 and 5 for back and forward as we think of the buttons.
So in Button 4 (back button) you want to, from the drop down menu, select "Input Shortcut Key" and check the ⌘ box and then type [
This will look like cmd + [ in the middle window. Then click ok.
Then in Button 5 (forward button) you want to, from the drop down menu, select "Input Shortcut Key" and check the ⌘ box and then type ]
This will look like cmd + ] in the middle window. Then click ok.
I realize this is totally non-obvious as there was no instruction that came with Steermouse but this did it for me! Have a great day.