1. 在VBA代码中添加以下函数：

Function Base64Decode(ByVal strData As String) As Byte() Dim objXML As MSXML2.DOMDocument Dim objNode As MSXML2.IXMLDOMElement

'创建DOMDocument对象
Set objXML = New MSXML2.DOMDocument

'使用Base64解码方法将字符串转换成byte数组
Set objNode = objXML.createElement("b64")
objNode.DataType = "bin.base64"
objNode.Text = strData
Base64Decode = objNode.nodeTypedValue

'销毁对象
Set objNode = Nothing
Set objXML = Nothing

End Function

2. 调用函数并将解码后的XML字符串传递给XML DOM对象进行解析。以下是示例代码：

Sub DecodeXML() Dim strXML As String Dim objXML As MSXML2.DOMDocument

'Base64编码后的XML字符串
strXML = "PHJlcXVlc3Q+PG5hbWU+ZW1wbG95ZWU8L25hbWU+PC9yZXF1ZXN0"

'解码Base64并将XML字符串传递给DOMDocument对象
Set objXML = New MSXML2.DOMDocument
objXML.async = False
objXML.LoadXML (Base64Decode(strXML))

'解析XML文档
'...

'销毁对象
Set objXML = Nothing

End Sub

注意：在执行此代码之前，需要将Microsoft XML, v6.0 引用添加到VBA项目中。