Assuming a userform combobox, on a form with a combo box a text box and two command buttons then in an ordinary module use the following code. This demonstrates how to add items from contacts to a combo box and write the first column of the selected item to the text box.

Option Explicit

Sub ShowMyForm()
Dim olNS As NameSpace
Dim olFolder As MAPIFolder
Dim olContact As ContactItem
Dim oFrm As New UserForm1
Dim strItem As String
Dim i As Long
    Set olNS = GetNamespace("MAPI")
    Set olFolder = olNS.GetDefaultFolder(olFolderContacts)
    On Error Resume Next
    With oFrm
        With .ComboBox1
            i = 1
            .AddItem "[Select Contact]"
            For Each olContact In olFolder.Items
                strItem = olContact.FileAs
                strItem = Replace(strItem, vbCrLf, Chr(32) & Chr(47) & Chr(32))
                .AddItem
                .List(i, 0) = strItem
                .List(i, 1) = olContact.FirstName
                .List(i, 2) = olContact.LastName
                i = i + 1
            Next olContact
            .ListIndex = 0
        End With
        .Show
        If .Tag = 0 Then GoTo lbl_Exit
        'do something with the values from the form.
    End With
lbl_Exit:
    Unload oFrm
    Set olNS = Nothing
    Set olFolder = Nothing
    Set olContact = Nothing
    Exit Sub
End Sub
and in the userform code
Option Explicit

Private Sub ComboBox1_Change()
    If Me.ComboBox1.ListIndex > 0 Then
        Me.TextBox1.Text = Me.ComboBox1.Column(0)
    Else
        Me.TextBox1.Text = ""
    End If
End Sub

Private Sub CommandButton1_Click()
    Me.Hide
    Me.Tag = 1
End Sub

Private Sub CommandButton2_Click()
    Me.Hide
    Me.Tag = 0
End Sub