This will do it.
Private Sub UserForm_Initialize()
Dim List As New Collection
Dim rnArea As Range, rnCell As Range
Dim vaValues As Variant
Set rnArea = ActiveSheet.Range(Range("C1"), Range("C65536").End(xlUp))
On Error Resume Next
For Each rnCell In rnArea
List.Add rnCell.Value, CStr(rnCell.Value)
Next rnCell
On Error GoTo 0
For Each vaValues In List
UserForm1.ListBox1.AddItem vaValues
Next
End Sub