Found 2 DirectInput devices:

0: 'Keyboard' axes: 0, hats: 0, buttons: 256 (buffered)
   1: 'Mouse' axes: 3, hats: 0, buttons: 8 (buffered)

So let's get this straight:
Left, Right, and Middle clicks work.
Wheel Up and Wheel Down work.
X/Y axis doesn't seem to work yet.

src/ScreenDebugOverlay.cpp

@@ -163,7 +163,7 @@ static LocalizedString DEBUG_MENU( "ScreenDebugOverlay", "Debug Menu" );
 void ScreenDebugOverlay::Init()
 {
 	Screen::Init();
-	
+
 	// Init debug mappings
 	// TODO: Arch-specific?
 	{
@@ -539,7 +539,6 @@ void ChangeVisualDelay( float fDelta )
 	pRet->Set( fSecs );
 }
 
-
 // DebugLines
 static LocalizedString AUTO_PLAY		( "ScreenDebugOverlay", "AutoPlay" );
 static LocalizedString ASSIST			( "ScreenDebugOverlay", "Assist" );
@@ -579,7 +578,6 @@ static LocalizedString SONG			( "ScreenDebugOverlay", "Song" );
 static LocalizedString MACHINE			( "ScreenDebugOverlay", "Machine" );
 static LocalizedString SYNC_TEMPO		( "ScreenDebugOverlay", "Tempo" );
 
-
 class DebugLineAutoplay : public IDebugLine
 {
 	virtual RString GetDisplayTitle() { return AUTO_PLAY.GetValue() + " (+Shift = AI) (+Alt = hide)"; }