The only callback signature I'm familiar with for onAction is something like
this C# signature:

public void Ribbon_onAction(Ribbon.IRibbonControl control)

{

}

For getPressed it looks like this:

public bool Ribbon_getPressed(IRibbonControl control)

{

}

To me that implies that for C++ it could be something like this:

CheckboxClicked([in]IDispatch* ribboncontrol, [out]VARIANT_BOOL state);

Are you sure about that [in] attribute on the state variable?

You can take a look at www.outlookcode.com, there are some C++ COM addin
samples for Outlook there, if you haven't seen them already.

For my own coding I restrict managed code addins to those that only need to
support Outlook 2003 or later. I could do 2000 or later but there are some
problems with the PIA's for 2002 and the 2002-modified one for 2000 that I
prefer to avoid. For addins that must support Outlook 2000 and later I use
VB6, I don't really do any C++ programming, just some hacking from time to
time.

When I need to do one of those addins I reference the Outlook and Office
2000 tlb's and add ribbon support by using a special XLIRibbonExtensibility
tlb that has the ribbon interfaces. That way I fork my code based on Outlook
version and use the CommandBars interface for versions before 2007 and the
ribbon for 2007 and 2010 for Inspectors (and Explorers for 2010).

When I do a C# addin that has to support Outlook 2003 and later I use ribbon
interface definitions based on a blog article by Andrew Whitechapel of the
VSTO team in a RibbonInterop class and handle the callbacks in a RibbonX
class. The RibbonInterop class has definitions like this (it's been modified
to also support the new interfaces in IRibbonUI for Office 2010):

//internal namespace to facilitate "Office" alias

namespace RibbonInterop

{

namespace Office

{

#region Ribbon

[ComImport(), Guid("000C0395-0000-0000-C000-000000000046"),
TypeLibType((short)0X1040)]

public interface IRibbonControl

{

[DispId(1)]

string Id { [return: MarshalAs(UnmanagedType.BStr)]

[MethodImpl(MethodImplOptions.InternalCall,

MethodCodeType = MethodCodeType.Runtime), DispId(1)] get; }

[DispId(2)]

object Context { [return: MarshalAs(UnmanagedType.IDispatch)]

[MethodImpl(MethodImplOptions.InternalCall,

MethodCodeType = MethodCodeType.Runtime), DispId(2)] get; }

[DispId(3)]

string Tag { [return: MarshalAs(UnmanagedType.BStr)]

[MethodImpl(MethodImplOptions.InternalCall,

MethodCodeType = MethodCodeType.Runtime), DispId(3)] get; }

}

[ComImport(), Guid("000C0396-0000-0000-C000-000000000046"),
TypeLibType((short)0X1040)]

public interface IRibbonExtensibility

{

[return: MarshalAs(UnmanagedType.BStr)]

[MethodImpl(MethodImplOptions.InternalCall, MethodCodeType =
MethodCodeType.Runtime), DispId(1)]

string GetCustomUI([In(), MarshalAs(UnmanagedType.BStr)] string RibbonID);

}

[ComImport(), Guid("000C03A7-0000-0000-C000-000000000046"),
TypeLibType((short)0X1040)]

public interface IRibbonUI

{

// for both Office 2007 and 2010

[MethodImpl(MethodImplOptions.InternalCall, MethodCodeType =
MethodCodeType.Runtime), DispId(1)]

void Invalidate();

[MethodImpl(MethodImplOptions.InternalCall, MethodCodeType =
MethodCodeType.Runtime), DispId(2)]

void InvalidateControl([In(), MarshalAs(UnmanagedType.BStr)] string
ControlID);

// for Office 2010

[MethodImpl(MethodImplOptions.InternalCall, MethodCodeType =
MethodCodeType.Runtime), DispId(3)]

void InvalidateControlMso([In(), MarshalAs(UnmanagedType.BStr)] string
ControlID);

[MethodImpl(MethodImplOptions.InternalCall, MethodCodeType =
MethodCodeType.Runtime), DispId(4)]

void ActivateTab([In(), MarshalAs(UnmanagedType.BStr)] string ControlID);

[MethodImpl(MethodImplOptions.InternalCall, MethodCodeType =
MethodCodeType.Runtime), DispId(5)]

void ActivateTabMso([In(), MarshalAs(UnmanagedType.BStr)] string ControlID);

[MethodImpl(MethodImplOptions.InternalCall, MethodCodeType =
MethodCodeType.Runtime), DispId(6)]

void ActivateTabQ([In(), MarshalAs(UnmanagedType.BStr)] string ControlID,

[In(), MarshalAs(UnmanagedType.BStr)] string Namespace);

}

#endregion

}

}