Use a folder dialog rather than a file name dialog as suggested.
Get the function from the referenced thread.
'http://www.vbaexpress.com/forum/showthread.php?t=34695
Private Sub CommandButton1_Click()
Dim myFile As String
On Error GoTo ErrorHandler
If ComboBox1.Text = "apple" Then
myFile = Get_Folder("Select Folder", ThisWorkbook.Path) & "\" & ComboBox1.Text & ".txt"
End If
If Dir(myFile) = "" Then Exit Sub
Workbooks.OpenText Filename:= _
myFile, Origin _
:=xlWindows, StartRow:=1, DataType:=xlFixedWidth, FieldInfo:=Array(Array _
(4, 1), Array(10, 1), Array(26, 1), Array(27, 1), Array(50, 1), Array(58, 1)), _
TrailingMinusNumbers:=True
'ActiveSheet.Move After:=Workbooks("retriveFile.xls").Sheets(1)
'ActiveWindow.WindowState = xlMaximized
ErrorHandler:
MsgBox "Please select a file", vbInformation, "unable to continue" '& Err.Number & vbCrLf & vbCrLf & Err.Description
End Sub