Working with current open email

I don't know exactly what's wrong with your code. For one thing, though, you are not validating that a new, editable email is even open. The following proof-of-concept does exactly what I think you're looking to do: insert some text into the active email being composed. If this is not possible it displays a message box explaining why.

The portion that inserts text will only work if Word is being used as the email editor (which will ALWAYS be the case in Outlook 2010+). If it is not you will have to parse and update the Body or HTMLBody text directly.

Sub InsertText()
    Dim myText As String
    myText = "Hello world"

    Dim NewMail As MailItem, oInspector As Inspector
    Set oInspector = Application.ActiveInspector
    If oInspector Is Nothing Then
        MsgBox "No active inspector"
    Else
        Set NewMail = oInspector.CurrentItem
        If NewMail.Sent Then
            MsgBox "This is not an editable email"
        Else
            If oInspector.IsWordMail Then
                ' Hurray. We can use the rich Word object model, with access
                ' the caret and everything.
                Dim oDoc As Object, oWrdApp As Object, oSelection As Object
                Set oDoc = oInspector.WordEditor
                Set oWrdApp = oDoc.Application
                Set oSelection = oWrdApp.Selection
                oSelection.InsertAfter myText
                oSelection.Collapse 0
                Set oSelection = Nothing
                Set oWrdApp = Nothing
                Set oDoc = Nothing
            Else
                ' No object model to work with. Must manipulate raw text.
                Select Case NewMail.BodyFormat
                    Case olFormatPlain, olFormatRichText, olFormatUnspecified
                        NewMail.Body = NewMail.Body & myText
                    Case olFormatHTML
                        NewMail.HTMLBody = NewMail.HTMLBody & "<p>" & myText & "</p>"
                End Select
            End If
        End If
    End If
End Sub

            '
            Dim myOlExp As Outlook.Explorer
            Dim myOlSel As Outlook.Selection

            Set myOlExp = Application.ActiveExplorer
            Set myOlSel = myOlExp.Selection
            'MsgBox myOlSel.item(1)


            Dim selectedFolder As Outlook.MAPIFolder
              Set selectedFolder = myOlExp.CurrentFolder
            Dim itemMessage As String
              itemMessage = "Item is unknown."


             Dim expMessage As String
             expMessage = "Your current folder is " & selectedFolder.Name & "." & vbCrLf

             If myOlSel.Count > 0 Then

                             Dim selObject As Object
                            Set selObject = myOlSel.item(1)

                            If (TypeOf selObject Is Outlook.mailItem) Then
                                Dim mailItem As Outlook.mailItem
                                Set mailItem = selObject
                                itemMessage = "The item is an e-mail message." & " The subject is " & mailItem.Subject & "."
                                mailItem.Display (False)

                            ElseIf (TypeOf selObject Is Outlook.contactItem) Then
                                Dim contactItem As Outlook.contactItem
                                Set contactItem = selObject
                                itemMessage = "The item is a contact." & " The full name is " & contactItem.Subject & "."
                                contactItem.Display (False)

                            ElseIf (TypeOf selObject Is Outlook.AppointmentItem) Then
                                Dim apptItem As Outlook.AppointmentItem
                                Set apptItem = selObject
                                itemMessage = "The item is an appointment." & apptItem.Subject & "."

                            ElseIf (TypeOf selObject Is Outlook.taskItem) Then
                                Dim taskItem As Outlook.taskItem
                                Set taskItem = selObject
                                itemMessage = "The item is a task." & " The body is " & taskItem.Body & "."
                            ElseIf (TypeOf selObject Is Outlook.meetingItem) Then
                                Dim meetingItem As Outlook.meetingItem
                                Set meetingItem = selObject
                                itemMessage = "The item is a meeting item. " & "The subject is " & meetingItem.Subject & "."
                            End If
                        End If
                        expMessage = expMessage & itemMessage
                    MsgBox (expMessage)
            End Sub

Do you mean the currently selected message? In that case you need to use Application.ActiveExplorer.Selection collection, not Application.ActiveInspector.CurrentItem.