The Virtual-Key Codes page you linked to is for Windows CE 5.0 and is missing some VK_'s.
Try Virtual-Key Codes (Windows)
And List of Virtual-Key Codes
I assume you've already tried VK's 5 and 6; VK_XBUTTON1 0x05, VK_XBUTTON2 0x06.
I have an AutoIt script that I used for a mouse that was doing weird things when R/L clicking.
For $i=0 To 255
If _IsPressed($i) Then
MsgBox(0,"Key Pressed - "&Chr( $i ),"ASCII = '"& $i &"'")
Here it is compiled:http://localhostr.com/file/JR1z1aF/VKM.zip