Class and Server.MapPath

I have created a class which makes a database connection. I receive an error when I use Server.MapPath and I don't understand this. Why can this bed? Do I have to import a namespace or ...?

Code:

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Imports System.Data
Imports System.Data.OleDb
Public Class clsMedieroer : Inherits clsMateriale
    Private _medie As Integer
    Private _medie1 As Integer

    Sub New(ByVal s As Integer, ByVal m As String)
        MyBase.New(m)
        Medie = s
    End Sub

    Public Sub FindMedie()
        Dim myDataset As New DataSet()

        Dim strConn As String = "Provider=Microsoft.Jet.OLEDB.4.0;" & _
        "Data Source=" & server.mappath("exceltest.xls") & ";" & _
        "Extended Properties=Excel 8.0;"

        Dim myData As New OledbDataAdapter("SELECT * FROM [Sheet1$] WHERE Id=2", strConn)
        myData.TableMappings.Add("Table", "ExcelTest")

   
        Select Case _medieroer
            Case Is = "St. 37.0 Konti"
                _medie1 = 15
            Case Else
                _medie1 = _medie
        End Select
    End Sub

    Public ReadOnly Property Medie1() As Integer
        Get
            Return _medie1
        End Get
    End Property

    Public WriteOnly Property Medie() As Integer
        Set(ByVal Value As Integer)
            _medie = Value
            FindMedie()
        End Set
    End Property

End Class

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System.Web.HttpContext.Current.Server.MapPath